feat: session fork for alternative fix paths

[dougss]

Summary

When REVIEW gives REQUEST_CHANGES, fork the session to try an alternative approach from the same checkpoint instead of retrying linearly. Increases success rate for complex fixes.

Current State

• Review loop retries linearly: same executor, same approach, just different error context
• No branching/forking of execution paths
• If the approach is fundamentally wrong, retries waste budget

Plan

1. Create src/core/session-fork.ts:
   • forkSession(repoPath, snapshotRef) — create parallel worktree from snapshot
   • mergeBestResult(forks[]) — pick the fork that passes validation
2. On REQUEST_CHANGES, fork: one path with review feedback, one with rephrased prompt
3. Run both in parallel (respects max_parallel_delegate)
4. Merge the winning fork back

Dependencies

• Requires snapshot system (#XX — git snapshot issue)
• Requires parallel delegate support

Files

• src/core/session-fork.ts (NEW)
• src/core/pipeline.ts
• src/integrations/git/worktrees.ts