feat(mcp): add coordination primitives — tamper-evident locks, presence, job hygiene

Pure, unit-tested modules for the next round of multi-agent coordination fixes
(hooks wired separately):

- lock-integrity: content-fingerprint helpers so a lock holder can detect when a
  locked file was changed out from under them (advisory locks made tamper-evident).
- agent-presence: a roster tracking online/idle agents so workers started before
  jobs are posted are visible to the orchestrator (claim can return WAITING+roster).
- job-hygiene: reclaim abandoned in_progress jobs back to the board after a TTL,
  so stale claims (e.g. months-old rot) stop silently blocking work.

22 new unit tests, all green; tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>