feat(artifacts): capture Claude Docs docs and session documents, claim unowned rows

- A Claude Docs `batch` that creates a doc is captured like an `Artifact`
  publish, titled by the doc's name and linked by its own frame URL.
- At Stop/SessionEnd, each pptx/docx/xlsx/pdf/html a workspace session
  created or changed is snapshotted into its owner's library (`auto`);
  an unchanged file is skipped, a changed one refreshes its snapshot.
- Server mode: a page with no owner carries a suggested owner (the
  workspace or home its session ran in). The suggested user claims or
  declines it; an admin claims or assigns any. New tab Unowned /
  Is this yours? on the Artifacts page; all three actions audited.

docs/API.md

@@ -3087,7 +3087,7 @@ A snapshot freezes a session as it is at that moment into one immutable `session
 ### Artifacts hub
 
 ```http
-GET    /api/artifacts?scope=all|mine|hub&kind=page|file|skill|plugin&q=   {items} — each with `installed` (default scope `all`)
+GET    /api/artifacts?scope=all|mine|hub|unowned&kind=page|file|skill|plugin&q=   {items} — each with `installed` (default scope `all`)
 GET    /api/artifacts/settings                 {serverMode, canShare, installsEnabled, canManage}
 PUT    /api/artifacts/settings                 {installsEnabled: boolean} — admin (any caller in local mode); the UI is Settings → AI tools → Artifacts Hub
 GET    /api/artifacts/candidates?laneId=       {items: [{path, sessionId, at, exists, pinned}]}
@@ -3107,17 +3107,21 @@ GET    /api/artifacts/:id/raw?path=&download=1 one snapshot file (sandbox CSP, n
 POST   /api/artifacts/:id/install              {replace?} → {installed, replaced?}
 POST   /api/artifacts/:id/uninstall            → {uninstalled, paths}
 GET    /api/artifacts/:id/installs             {items: [{user, mine, at, replaced, paths?}], unrecorded}
+POST   /api/artifacts/:id/claim                → {item} — the suggested user, or an admin
+POST   /api/artifacts/:id/decline              → {id, declined} — the suggested user
+POST   /api/artifacts/:id/assign               {userId} → {item} — admin
 ```
 
 What agents produced, per OWNER (`server/lib/artifacts.js`, plan: `docs/plans/artifacts/README.md`):
 
-- **page** — a claude.ai link (`https://claude.ai/…` only, `400 EBADURL`), one row per owner and link. A session's `Artifact` tool publish is captured from its PostToolUse hook (the first `https://claude.ai/…artifact/…` in `tool_response`), owned by the session's workspace owner — or, for a session no workspace owns (a member's `claude` started from their own terminal under S3), by the member whose account sent the hook (by its connection's uid, never the body); the host account names nobody, so such a page from the host stays unowned.
-- **file** — `candidates` are the documents (html, md, pdf, images, csv, office…) the workspace's sessions wrote with Write/Edit/MultiEdit/NotebookEdit, inside its folder. A pin is read under the download route's guards (`lanePath`, `resolveInLane`, `ESECRET`, ≤ 25 MB, `413 ETOOBIG`) and SNAPSHOTTED to `<data dir>/artifacts/<random>/`.
+- **page** — a claude.ai link (`https://claude.ai/…` only, `400 EBADURL`), one row per owner and link. A session's `Artifact` tool publish — or a Claude Docs `batch` that creates a doc (`container.create`; its title is the doc's name, later batches are skipped) — is captured from its PostToolUse hook (a doc's own `frame.artifactUrl`; for a publish the first `https://claude.ai/…artifact/…` in `tool_response`), owned by the session's workspace owner — or, for a session no workspace owns (a member's `claude` started from their own terminal under S3), by the member whose account sent the hook (by its connection's uid, never the body); the host account names nobody, so such a page from the host stays unowned, with `suggested_owner_id` set to the owner of the workspace whose folder holds the session's `cwd` (else, under S3, the member whose home it is).
+- **file** — `candidates` are the documents (html, md, pdf, images, csv, office…) the workspace's sessions wrote with Write/Edit/MultiEdit/NotebookEdit, inside its folder. **Captured at Stop:** when a session a workspace owns sends `Stop` or `SessionEnd`, each `.pptx`/`.docx`/`.xlsx`/`.pdf`/`.html` in the workspace it created or changed since it started (a repo's untracked or modified files — an HTML file only when untracked — or a plain folder's files; at most 20 per stop, ≤ 25 MB) is snapshotted into the workspace owner's library as `auto: true` (shown **Auto**), off the hook's response path. The same file at the mtime its snapshot recorded is skipped; a changed one replaces that row's snapshot; a file pinned by hand is left alone. A pin is read under the download route's guards (`lanePath`, `resolveInLane`, `ESECRET`, ≤ 25 MB, `413 ETOOBIG`) and SNAPSHOTTED to `<data dir>/artifacts/<random>/`.
 - **skill** — the caller's own `~/.claude/skills/<name>` (their member home under S3; the host's for an admin or in local mode, else `409 ENOACCOUNT`) or a workspace's `.claude/skills/<name>`. Snapshotted whole: ≤ 300 files, ≤ 5 MB, a symlink anywhere is `ESYMLINK`, a secret-looking file name (`.env`, `*.pem`, `*token*`…) `403 ESECRET`.
 - **file upload** — one file from the user's computer (≤ 25 MB, `413 ETOOBIG`), snapshotted under its own base name (no folder kept).
 - **skill upload / new** — a skill made from files rather than picked: one Markdown/text file of any name becomes `SKILL.md`; several files or a picked/dropped folder (`paths` keeps the layout) need a `SKILL.md` (at the top or in one folder, which is stripped — `400 ENOSKILLMD` otherwise); a single `.zip`/`.skill` is unpacked (sizes checked from its directory first, a symlink entry `ESYMLINK`, `__MACOSX`/`.DS_Store`/`.git`/`node_modules` skipped); an upload holding several skill folders makes one artifact per skill, named by its folder, and stores NOTHING if any of them fails. The `name` the user typed (only for a single skill; else the frontmatter `name:`, the folder or the file name) is made a folder name (`skillSlug`: marks dropped, other characters → `-`; `400 EBADNAME` if nothing is left) and written into `SKILL.md`'s frontmatter as `name:`; a `description` given replaces the frontmatter's, and a skill with none anywhere is `400 ENODESCRIPTION`. Paths are relative with no `..` (`EBADPATH`); the same limits and `ESECRET` as a picked skill. `new` writes `SKILL.md` from `content` the same way.
 - **plugin** — only the spec and its marketplace source (`EBADPLUGIN`, `EBADMARKET`: `owner/repo`, a `validateRemoteUrl` URL, or an https link to a `marketplace.json`); no files. A GitHub web link becomes `owner/repo`, and a forge's `…/src/…`, `…/tree/…`, `…/blob/…` suffix or trailing slash is trimmed. The same `key` added again by its owner updates that row. **inspect** reads the marketplace's `.claude-plugin/marketplace.json` to list its plugins: a git link is cloned shallow, blob-less and without checkout into a temp dir that is removed after, with every credential helper switched off and no prompt (a repository that needs a login is `502 ENEEDLOGIN`; type the key by hand — installing then uses each user's own git login), https only (an ssh link is `400 ESSHMARKET`); a repo with only a `plugin.json` is `400 ENOTMARKET`; unreachable `502 EMARKETFETCH`. It reaches any https host a signed-in user names, like `publish` does — no login is ever sent with it.
 - **Visibility** — local mode: every row, no sharing (`400 ELOCALMODE`). Server mode: a developer sees their own rows and shared ones, admin/analyst every row; only the owner or an admin changes or deletes one (`403 EFORBIDDEN`). `shared` puts a row in every user's **Hub**. The page shows one list (`scope=all`: own rows plus shared ones) with an **All · My library · Hub** filter. `scope=mine` is the caller's own library rows plus the shared skills/plugins of OTHER users that the caller has installed (`installed: true`, `mine: false`); in server mode the page splits EVERY tab by `mine`, others' section first — **All** into *Shared by others* / *Mine*, **Hub** into *Shared by others* / *Shared by me*, **My library** into *Installed from others* / *Mine* — and each section pages 12 cards at a time (client-side). Deleting a SHARED row of your own only takes it out of your library (`in_library = 0`, `inLibrary: false`, `{kept: true}`; it leaves `scope=mine` and shows as **Hub only**); unsharing such a row then deletes it (`PATCH` answers `{item, removed: true}`). Deleting an unshared row, or an admin deleting someone else's, deletes it outright.
+- **Unowned** — server mode: a row with no owner (`unowned: true`). `scope=unowned` lists every one for an admin and, for anyone else, those suggested to them (`suggestedForMe: true`; such a row is also visible to them, `canSeeId`). **claim** makes the suggested user — or an admin — its owner; **decline** drops the suggestion (the row stays for an admin); **assign** `{userId}` is an admin giving it to an enabled user (`400 EBADFIELD` otherwise). All three answer `409 EOWNED` once it has an owner, `400 ELOCALMODE` in local mode, and are written to `audit_log`. The page shows the tab **Unowned** to an admin and **Is this yours?** to a user with suggestions.
 - **raw** — only a file the snapshot recorded (`404 ENOFILE` otherwise), never a request path; served with `Content-Security-Policy: …; sandbox` and `nosniff`, unknown types as an attachment.
 - **install** — a skill or plugin (`EBADKIND`), into the CALLER's own account: a skill is written to `~/.claude/skills/<name>/` AND `~/.agents/skills/<name>/` (the folder Codex, Gemini/Antigravity, OpenCode and Cursor read) of every account the caller's sessions run as — under S3 their member home (as them through `write-as`), plus the host's home for an admin, whose host-folder workspaces run as the host. The answer is `{installed, paths, replaced}` (`installed` = the paths joined). A name taken in any of them is `409 EEXISTS` unless `replace: true`, which moves each old one to `~/.claude/agenthub-replaced-skills/<name>[-agents]-<time>/`. A plugin runs `claude plugin marketplace add <market>` then `claude plugin install --scope user <key>` as the caller, never with `-y`. `403 EINSTALLSOFF` while an admin has turned installs off. A list row's `installed` is true when that skill (`~/.claude/skills/<name>`) or plugin (a key of `~/.claude/plugins/installed_plugins.json`) is already in EVERY one of those accounts — the page then shows **Installed** instead of Install; it is a pure read, and an account that cannot be resolved or read counts as not installed. Every install is also a row of `artifact_installs` (who, when, the folders written, whether it replaced); `GET …/installs` lists them newest first — the owner, staff and local mode see every install, anyone else only their own, and `paths` goes only to the installer and an admin (a plugin has none: it is `claude plugin install --scope user`). `unrecorded` is what `installs` counted before that table existed. The card's "N installs" opens this list. Share, unshare, a shared row's delete and every install or uninstall by someone else are written to `audit_log`.
 - **uninstall** — takes an installed skill or plugin out of the same accounts again (`EBADKIND` otherwise; allowed while installs are off). A skill's folders that exist (both skills folders of each account) are MOVED to `~/.claude/agenthub-replaced-skills/<name>[-agents]-uninstalled-<time>/`, never deleted; a plugin runs `claude plugin uninstall --scope user --keep-data <key>` as the caller (its data dir stays). Nothing of it there is `409 ENOTINSTALLED`. The answer is `{uninstalled, paths}` (`paths` = the skill folders moved; empty for a plugin). The `installs` count and `artifact_installs` history are left as they were. The page offers **Uninstall** only in the card's ⋯ menu (and its right-click menu); there is no Reinstall — a newer version is uninstalled and installed again.