feat(mcp): tamper-evident locks, agent presence/idle-claim, stale-job reclaim, actionable denials

Wires the coordination primitives into the live tools, addressing the rest of the
multi-agent field report:

- Tamper-evident locks: locks now record a content fingerprint; new verify_file_lock
  tool lets a holder confirm a file wasn't rewritten under their (advisory) lock
  before overwriting it — the realistic defense against "edited, then clobbered".
- Agent presence: every tool call updates a roster; new list_agents tool plus
  claim_next_job returning WAITING+roster (instead of dead NO_JOBS) so workers
  started before jobs are posted are visible — users no longer wait on the orchestrator.
- Stale-job reclaim: claim_next_job returns abandoned in_progress jobs (idle past the
  lock timeout) to the board so months-old rot stops blocking work.
- Actionable denials: REQUIRES_ORCHESTRATION/REJECTED now name who holds the lock,
  their intent, and how to recover (other file/job, update_shared_context, expiry,
  force_unlock) instead of a vague "locked by another agent".

Integration tests cover verify (OK/CONFLICT/NO_LOCK/UNKNOWN), reclaim, and denial
messaging; dist rebuilt. Full suite 71/71 green, tsc + eslint clean.

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