## Why
Older Desktop clients can still provide bundled Sites to an independently updated SSH app-server. A cached remote Sites install must take precedence, even when disabled, while a missing remote bundle must preserve the bundled fallback.
## What changed
- Remove persisted bundled-plugin exclusions, Sites migration checks, and the migration wait when loading local plugin configuration.
- Suppress `sites@openai-bundled` during plugin loading when the remote global catalog is active and a cached remote Sites install is available.
- Remove exclusion-based catalog filtering and read/install guards, and simplify `install_plugin` to accept `ConfigLayerStack`.
## Testing
Expand the agent-turn Sites test to cover enabled remote precedence, disabled remote suppression of bundled Sites, and fallback when the remote bundle is missing.
GitOrigin-RevId: e2758596e5493ccee051cdfbf6b4afbe4f98948c