Make replay cleanup interruption-safe

[kitlangton]

Summary

• Ensure Git.replay always aborts in-progress cherry-picks, restores the original branch, and deletes the temporary replay branch.
• Add regression coverage for replay failure cleanup.

Verification

• bun run typecheck
• bun run test
• bun src/cli.ts --help
• bun src/cli.ts sync --help
• bun src/cli.ts merge --help

Stack

1. Make replay cleanup interruption-safe #1 👈 current
2. Use Effect clock for stack timestamps #2
3. Use schema-backed domain errors #3
4. Tighten typed error channels #4
5. Configure GitHub polling and concurrency #5