Removes `pre_delegation`, `retry_attempt`, `flow_transition`, and
`router_decision` — none has a hard use case yet, and `retry_attempt`
was wired at three unrelated seams (agent task retry, tool parse retry,
guardrail retry) with a different payload at each, so its semantics were
ambiguous. Also casts the `post_step` payload back to `TaskOutput` in
`task.py` so the seam keeps mypy green.
356215a9f7