Base selection and checkout provenance

[BramVR]

Parent

#70

What to build

Implement the accepted base selection order and record checkout provenance in the Agent Run Contract. Agent Prep should choose explicit base, then project policy, then discoverable remote default, then main fallback, and prove the fetched and prepared commits in metadata.

Acceptance criteria

• Base selection follows explicit request, project policy, remote default, then main fallback.
• Missing or invalid base remains a readiness blocker.
• Agent Run Contract records fetched base, fetched commit, prepared HEAD, and whether they match.
• Tests cover non-main remote default behavior and policy override behavior.
• e2e proves an agent handoff does not silently start from the wrong branch.

Blocked by

• Git Operations and Project identity locality #71
• Shared Readiness Decision for existing commands #72
• Agent Run Contract v1 baseline #73