fix(updates): report a plugin build installed beside the running one, never "up to date" on a failed check

A `claude plugin update` run from a terminal (or with only the Claude desktop
app open) unpacks the new build and starts nothing: no SessionStart fires, so
the dashboard kept running the old cache dir across reboots, and with the
forge unreachable the sidebar read "Up to date" (seen on a machine stuck at
1.8.10).

- pluginStatus asks supersededBy for its own root: a recorded sibling install
  sets `installed_not_running: {root, version}` and `update_available`, with
  no network needed
- the sidebar reads "<version> installed, not running"; the update modal keys
  on it, so Update now is offered even with no release tag or sha
- plugin-update-helper hands over to the recorded install first, the newest
  sibling dir only as a fallback
- a plugin install whose release lookup failed reads "Could not check for
  updates" instead of "Up to date"
- docs/PLUGINS.md: the case and the manual hand-over command

client/src/i18n/locales/en/updates.json

@@ -26,5 +26,6 @@
   "pluginAutoNote": "This dashboard is installed as a Claude Code plugin. \"Update now\" pulls the new plugin version, reinstalls the runtime, and restarts the server for you — it takes a few minutes. Newly added skills and slash commands only appear in an already-open Claude Code session after you run /reload-plugins there.",
   "pluginUpdating": "Pulling the new plugin version and reinstalling the runtime — this takes a few minutes, the page reloads itself when the new server is up...",
   "lastPluginUpdateFailed": "The last plugin update failed at step \"{{step}}\": {{error}} — run /agenthub-update in Claude Code to recover.",
-  "restartingShort": "Restarting…"
+  "restartingShort": "Restarting…",
+  "installedTitle": "Version {{version}} is installed but not running"
 }