fix: cache trail enablement off the prompt path

Stop probing the trails API from TurnStart. Cache server-side trail enablement in clone-local preferences (.git/entire/preferences.json) during the existing entire enable backend report, then make model-context injection read only that local cache.

This keeps the prompt path free of git subprocesses, auth discovery/token exchange, and HTTP requests. A missing/stale false cache fails closed (no hint) rather than blocking the agent or retrying every turn.