feat(artifacts): a shared skill follows its owner's folder; installers get Update

A skill in the Hub was a snapshot taken once: the owner changing
~/.claude/skills/<name> changed nothing there, and an installer had to
uninstall and install again to get a newer version.

- syncSkillSources (on GET /api/artifacts, at most every 15 s): a skill row
  taken from a folder — the owner's ~/.claude/skills/<name>, or the
  workspace's .claude/skills/<name> — is brought up to it. Only a folder
  whose names/sizes/mtimes changed is read again, with a pick's checks (no
  symlink, no secret, <= 5 MB, a member's files as theirs); a change is a
  NEW snapshot, the old one dropped, and an artifact_changed broadcast.
- artifacts.version (snapshot content hash) and source_hash (the folder's
  when last taken, additive columns): an upload or a Hub edit records the
  folder as it is then, so only a LATER folder change replaces it.
- A list row's updateAvailable: installed, and the viewer's copy differs
  from version. The page shows Update (a pack: Update N); install with
  {update: true} replaces the copies (old ones moved aside) and is not
  counted as another install.

docs/API.md

@@ -3119,7 +3119,7 @@ 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|unowned&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` and `updateAvailable` (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}]}
@@ -3136,7 +3136,7 @@ GET    /api/artifacts/:id                      {item}
 PATCH  /api/artifacts/:id                      {title?, description?, shared?, source?, market?, skill?: {name, description, content}}  (source/market: plugin only; skill: a skill's SKILL.md frontmatter name/description and body — written as a new snapshot, other files kept, row `source`/`description` follow)
 DELETE /api/artifacts/:id                      {ok, kept} — kept: a shared row of yours left your library but stays in the Hub
 GET    /api/artifacts/:id/raw?path=&download=1 one snapshot file (sandbox CSP, nosniff)
-POST   /api/artifacts/:id/install              {replace?} → {installed, replaced?}
+POST   /api/artifacts/:id/install              {replace?, update?} → {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
@@ -3155,7 +3155,8 @@ What agents produced, per OWNER (`server/lib/artifacts.js`, plan: `docs/plans/ar
 - **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`.
+- **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.
+- **a skill follows its source** — a skill row taken from a folder (its owner's `~/.claude/skills/<name>`, or the workspace's `.claude/skills/<name>`; the host's in local mode) is brought up to that folder whenever the Hub is listed (`syncSkillSources`, at most every 15 s; only a folder whose file names, sizes or mtimes changed is read again, through the same checks as a pick — no symlink, no secret, ≤ 5 MB — a member's files as theirs). A change is a NEW snapshot directory, then the old one dropped, and an `artifact_changed` broadcast. `artifacts.version` is a snapshot's content hash and `source_hash` the folder's when last taken: an upload or a Hub edit records the folder as it is then, so only a LATER change to the folder replaces it — the newer change wins. A list row's `updateAvailable` is true when the skill is `installed` and the viewer's `~/.claude/skills/<name>` (any account they run as) differs from `version`; the page then shows **Update**. `install` with `update: true` (a skill, `ENOTINSTALLED` when it is not installed) replaces those copies like `replace: true` — the old ones moved aside — and is not counted as another install. 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.
 - Every write is same-origin. The `artifact_changed` broadcast (`{id, ownerId, shared}`) reaches everyone for a shared row, else its owner and staff.