Adds a per-task claim status helper so callers can check 'is X
claimable right now?' and 'what's blocking X?' without trying to
claim and catching the exception.

- TaskService.getClaimStatus(taskId): returns { status, claimable,
  blocked_by: [{id, title, status, stage}] }. blocked_by lists the
  specific incomplete dependencies preventing claim.
- task_get default response now includes claim_status (no opt-in
  needed; the data is small).
- 6 unit tests covering isolated, single blocker, completed blocker,
  multiple blockers, cancelled-as-resolved, and in-progress status.
- bench/README.md: removed Critical pitfalls section. Reframed
  Limitations to document what's been addressed and to note the
  dep-aware-mgmt naive-baseline variance issue (questions leak task
  names which lets the LLM occasionally guess project structure).