fix(pi-herdr): degrade busy-agent status, brief-on-disk dispatch, gone-checkout cleanup

- herdr_task_status: catch agent_not_idle from the terminal read and return
  the lifecycle state with a 'transcript: unavailable (agent busy)' note
  instead of erroring; status-poll timeouts return state: unknown with the
  underlying message as a partial result.
- herdr_task: multi-line or >2000-char tasks are written to
  .pi-herdr-brief.md in the worktree and the agent starts with a short
  pointer prompt; invalid_agent_argument from agent start retries once
  with a brief file.
- herdr_task_cleanup: an already-deleted checkout (or one removed between
  the existence check and git worktree remove) is a successful cleanup
  with a note, not an error.