feat(artifacts): Share again takes a skill's newest version; a same-named upload updates it

A shared skill followed its owner's ~/.claude/skills folder only, so one
uploaded from a browser (or living in a plugin) stayed at its first snapshot
for good. Share again (a skill's ⋯ menu and detail sheet) now takes the
owner's copy on the server — its source folder, else the same-named skill of
a plugin they installed at user scope — and shares it; with no copy it asks
for the folder and uploads it onto that row (`id`). An upload whose skill is
named like one the uploader already has replaces that row's version instead
of adding a twin. Installers are then offered Update, as before.

docs/API.md

@@ -3156,7 +3156,7 @@ POST   /api/artifacts/page                     {url, title?, description?}
 POST   /api/artifacts/file                     {laneId, path, title?, sessionId?}
 POST   /api/artifacts/file/upload              multipart: files (one), title?, description? → {item}
 POST   /api/artifacts/skill                    {name, laneId?, title?, description?}
-POST   /api/artifacts/skill/upload             multipart: files, paths (JSON, each file's relative path), name?, title?, description? → {items}
+POST   /api/artifacts/skill/upload             multipart: files, paths (JSON, each file's relative path), name?, title?, description?, id? → {items} — a skill named like one the caller already has becomes its new version (`replaced`, `changed`); `id` = Share again onto that row
 POST   /api/artifacts/skill/new                {name, description, content, title?} → {item, items}
 POST   /api/artifacts/plugin/inspect           {market} → {market, name, description, plugins: [{name, key, description, version, category}]}
 POST   /api/artifacts/plugin                   {key: "name@marketplace", market: "owner/repo" | git URL | marketplace.json URL, title?, description?}
@@ -3166,6 +3166,7 @@ DELETE /api/artifacts/:id                      {ok, kept} — kept: a shared row
 GET    /api/artifacts/:id/raw?path=&download=1 one snapshot file (sandbox CSP, nosniff)
 POST   /api/artifacts/:id/install              {replace?, update?} → {installed, replaced?}
 POST   /api/artifacts/:id/uninstall            → {uninstalled, paths}
+POST   /api/artifacts/:id/reshare              → {item, changed} — Share again (skill, owner): the owner's folder on this server becomes the new version and the row is shared; 409 ENOSOURCE when the server has no copy
 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
@@ -3177,6 +3178,7 @@ What agents produced, per OWNER (`server/lib/artifacts.js`, plan: `docs/plans/ar
 - **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`, `id_rsa`, `.netrc`…, or a data file named `*token*`/`*secret*`/`*password*`/`*credential*` — the word alone does not count in Markdown, source code or images, e.g. `design-tokens.md`) `403 ESECRET`.
+- **Share again** (a skill's ⋯ menu and detail sheet, its owner, server mode) — `reshare` takes the owner's skill folder on THIS server as the new snapshot: the row's source folder, else the same-named `skills/<name>` of a plugin the owner installed at user scope (`installed_plugins.json`, the most recently updated one, never outside their `~/.claude/plugins/`), and shares the row; installers are then offered **Update**. With no copy on the server (a skill uploaded from a browser) it answers `409 ENOSOURCE` and the page asks for the skill's folder, uploaded with `id` onto that row. An upload whose skill is named like one the uploader already has (not a workspace's) also replaces that row's version rather than adding a twin.
 - **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.