Migrate to bt2 and unify per-repo write locking

Switch from bordeaux-threads v1 (bt:) to v2 (bt2:) for keyword-based
APIs and lock timeout support.  Unify Chamber RPC and SSH push write
locking through shared per-repo semaphores so web merges and SSH
pushes properly serialize against each other.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>