Sessions

Session lifecycle:

1. gittrix.startSession({ task, durable, ephemeral })
2. GitTrix captures durable baseline and creates an ephemeral fork.
3. Agent works with AgentSession (read/write/commit in ephemeral).
4. Human-facing UI holds UserSession.
5. User reviews diff against baseline.
6. User calls promote(...) for accepted changes.
7. Session evicts on policy.

Sessions have stable IDs and can be resumed by ID.