fix(mcp): claim_job honored, force_unlock distinct, grant-echo verified

Live sweep findings against the hosted API:

- claimJob sent action:'claim' — which the server treats as claim-NEXT,
  silently ignoring jobId. Agents asked for one job and got the head of
  the queue (reproduced live twice). Now sends 'claim_by_id' and recovers
  structured 404/409 bodies (NOT_FOUND / BLOCKED_BY_DEPENDENCIES).
- forceUnlock now sends a distinct 'force_unlock' action (with reason)
  so the server can scope plain 'unlock' deletes by agent_id; falls back
  to legacy 'unlock' on older deployed servers.
- proposeFileAccess verifies the server's GRANTED echo is attributed to
  this agent before writing on it (defense-in-depth).

Server-side counterparts in axis-frontend 0fb7d8c8. 4 new contract tests
with a fetch-capturing harness; suite 118 pass / 0 fail.

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