feat(artifacts): storefront layout and Uninstall for installed skills/plugins

- Artifacts page laid out like an app store: hero search, category chips,
  shelves by kind, app-icon cards with one primary pill and a detail sheet
- POST /api/artifacts/:id/uninstall: a skill's copies in ~/.claude/skills and
  ~/.agents/skills are moved to ~/.claude/agenthub-replaced-skills/
  (-uninstalled-<time>), never deleted; a plugin runs
  `claude plugin uninstall --scope user --keep-data` as the caller;
  ENOTINSTALLED when nothing is there; allowed while installs are off
- Uninstall on the card, the ⋯ menu and the detail sheet, after a confirm

docs/API.md

@@ -3050,6 +3050,7 @@ PATCH  /api/artifacts/:id                      {title?, description?, shared?, s
 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/uninstall            → {uninstalled, paths}
 GET    /api/artifacts/:id/installs             {items: [{user, mine, at, replaced, paths?}], unrecorded}
 ```
 
@@ -3063,7 +3064,8 @@ What agents produced, per OWNER (`server/lib/artifacts.js`, plan: `docs/plans/ar
 - **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. 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.
 - **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 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. 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 shows **Uninstall** (card icon, ⋯ menu, detail sheet) beside **Installed**.
 - Every write is same-origin. The `artifact_changed` broadcast (`{id, ownerId, shared}`) reaches everyone for a shared row, else its owner and staff.
 
 ### Query Explorer