fix(updates): check releases for a plugin install with no .git

An archive-sourced marketplace entry ("source": "url" — this project's own)
makes Claude Code unpack the plugin rather than clone it, so the cache dir
has no .git. getUpdatesStatus probed for .git first and returned "not a git
clone; check for updates manually", skipping the entire release layer:
update_available stayed false however many releases were published, and
plugin_marketplace/plugin_name were never set, so POST /api/updates/apply's
plugin branch could never fire (it answered 409, claiming nothing newer was
published). The bug reads as "up to date", so it stayed invisible for as
long as the install happened to be current.

The plugin branch now runs BEFORE the .git probe and answers without git at
all: the repository URL comes from the marketplace manifest on disk
(known_marketplaces.json -> .claude-plugin/marketplace.json -> source.url,
null rather than a guessed host for a github owner/repo shorthand), and
package.json is the only local comparison signal. A failed release lookup
is reported as release_check, never as "nothing newer".

UI/CLI: the "not a git clone" note is gated on install_kind !== "plugin", so
the Update now button is offered where the dashboard can actually check and
apply.

CLAUDE.md

@@ -1,6 +1,6 @@
 # Claude Code Working Guide
 
-_Last synced: eb63569 (2026-09-10) + release-based update detection (`server/lib/update-check.js` reads the forge releases API; a `plugin_managed` install self-updates via the detached `plugin-update` chain) + the install-wide **Update all** batch and session reload (`server/lib/cc-update-all.js`, `server/lib/cc-reload.js`)._ A change that makes any statement in this file, `AGENTS.md`, or a README wrong — a moved path, a new or changed extension point, a retired rule — updates that file in the same change-set and bumps this marker.
+_Last synced: eb63569 (2026-09-10) + release-based update detection (`server/lib/update-check.js` reads the forge releases API — a checkout via its `origin` remote, a plugin-cache install via the marketplace manifest on disk, since an archive-sourced cache dir has no `.git`; a `plugin_managed` install self-updates via the detached `plugin-update` chain) + the install-wide **Update all** batch and session reload (`server/lib/cc-update-all.js`, `server/lib/cc-reload.js`)._ A change that makes any statement in this file, `AGENTS.md`, or a README wrong — a moved path, a new or changed extension point, a retired rule — updates that file in the same change-set and bumps this marker.
 
 ## Project mission
 - Maintain a reliable local-first dashboard for Claude Code session monitoring.