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.

client/src/lib/api.ts

@@ -3143,6 +3143,13 @@ export const api = {
         method: "POST",
         body: JSON.stringify({ replace }),
       }),
+    /** POST /api/artifacts/:id/install `{update: true}` — an installed skill's
+     *  copies replaced by its newest version (the old ones moved aside). */
+    updateInstalled: (id: number) =>
+      request<{ installed: string; replaced?: boolean }>(`/artifacts/${id}/install`, {
+        method: "POST",
+        body: JSON.stringify({ update: true }),
+      }),
     /** POST /api/artifacts/:id/uninstall — a skill moved aside, a plugin `claude plugin uninstall`. */
     uninstall: (id: number) =>
       request<{ uninstalled: string; paths: string[] }>(`/artifacts/${id}/uninstall`, {
@@ -4423,6 +4430,9 @@ export interface Artifact {
   inLibrary: boolean;
   /** List rows only: a skill/plugin already in every account the viewer's sessions run as. */
   installed?: boolean;
+  /** List rows only: an installed skill whose copy differs from its newest
+   *  version (its owner changed it since) — the list offers Update. */
+  updateAvailable?: boolean;
   /** file: captured when its session stopped, not pinned by hand. */
   auto?: boolean;
   /** Server mode: nobody owns it yet. */