fix: correct marketplace.json source field and improve async operations

- Fix marketplace.json: change source from relative path to GitHub URL
  (was "skills/unraid", now "https://github.com/jmagar/unraid-mcp")
  This resolves the "Invalid input" schema validation error when adding
  the marketplace to Claude Code

- Refactor subscriptions autostart to use anyio.Path for async file checks
  (replaces blocking pathlib.Path.exists() with async anyio.Path.exists())

- Update dependencies: anyio 4.11.0→4.12.1, attrs 25.3.0→25.4.0

.claude-plugin/marketplace.json

@@ -13,7 +13,7 @@
   "plugins": [
     {
       "name": "unraid",
-      "source": "skills/unraid",
+      "source": "https://github.com/jmagar/unraid-mcp",
       "description": "Query and monitor Unraid servers via GraphQL API - array status, disk health, containers, VMs, system monitoring",
       "version": "0.2.0",
       "tags": ["unraid", "monitoring", "homelab", "graphql", "docker", "virtualization"],