Prefer remote Sites over the bundled plugin

[copyberry]

Prefer remote Sites over the bundled plugin

Why

When the remote Sites plugin is installed and its local bundle is available, exposing the bundled copy as well creates duplicate catalog entries and can load the wrong skills.

What changed

• Persist an account- and backend-scoped exclusion for sites@openai-bundled once the remote replacement is installed and loadable.
• Apply the exclusion to plugin catalogs, search, direct reads and installs, runtime loading, hooks, and skill caches while preserving the remote plugin's server-owned enabled state.
• Restore the bundled fallback when the remote plugin is absent or its local files are unavailable, and throttle repeated availability checks for 60 seconds.

Testing

• Added manager tests for persistence across restarts, account/backend isolation, fallback restoration, concurrent checks, and refresh throttling.
• Added app-server and agent-turn tests confirming that catalog, search, and skill loading prefer remote Sites.

[github-actions]

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