Dolt branch lifecycle: create player branch at session start, commit at session end

[theinterneti]

Part of #269

Implement DOLT_BRANCH('player/{player_id}/session-{id}') at session start (branching from main HEAD). DOLT_CHECKOUT to player branch. DOLT_COMMIT at session end before reconciliation.

Acceptance Criteria

• Session start: CALL DOLT_BRANCH('player/{player_id}/session-{session_id}') branching from main
• DOLT_CHECKOUT switches to player branch for duration of session
• All writes during session go to player branch (not main)
• Session end: DOLT_COMMIT with session summary message
• Integration tests: branch created, writes isolated, committed at end