fix(plugin): Correct Claude marketplace source path

Update the Claude marketplace entry to use a schema-valid relative source path and add a regression test so invalid marketplace sources fail in the local suite. Also document the maintainer workflow used for stale PR metadata and fork-gated Actions runs.

Fixes #344

.claude-plugin/marketplace.json

@@ -27,7 +27,7 @@
         "plugin",
         "marketplace"
       ],
-      "source": "."
+      "source": "./"
     }
   ]
 }