chore(plugin): add npm run check:plugin with explicit manifest paths

Runs the mirror/version drift check and both strict validations. The paths
are spelled out because the documented pair ended in a bare `.` that reads as
a full stop when copied out of prose, which surfaced as `missing required
argument 'path'` on first use. Needs the `claude` CLI, so it is a local check
rather than a CI step.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

package.json

@@ -38,6 +38,7 @@
     "changeset": "changeset",
     "version-packages": "changeset version && node scripts/sync-plugin.mjs && npm install --package-lock-only && node scripts/check-lockfile-sync.mjs",
     "check:lockfile": "node scripts/check-lockfile-sync.mjs",
+    "check:plugin": "node scripts/sync-plugin.mjs --check && claude plugin validate --strict plugins/codeman && claude plugin validate --strict .claude-plugin/marketplace.json",
     "knip": "npx --yes knip@latest --config config/knip.json",
     "release": "changeset publish"
   },