feat(gateway): developer i_will_work_on takes a substantive step checklist (#172)

The dev plan was a free string with only a presence gate, so the
executing dev had no checklist for plan-driven progress (#173).

- IWillWorkOnRequest gains `steps` (same SubTask shape as a PM's
  sub_tasks); flow_dev route threads it through.
- i_will_work_on layers steps onto the narrative plan via the same
  panel-shaped path PMs use, so task.plan.sub_tasks is populated
  (panel render + #173 progress).
- New _dev_steps_gate (mirrors _pm_sub_tasks_gate, runs after the spec
  gate): a developer FRESH claim must supply a non-empty steps list
  with every description >= _PM_SUBTASK_DESC_MIN_LEN. Re-entry/recovery
  short-circuit before the gate (extracted _dev_reentry +
  _fresh_dev_claim keep i_will_work_on within the
  return-count + cyclomatic gates).
- developer role prompt: steps template + "thin steps rejected" + the
  progress(plan_step=...) handoff.
- Updated every dev-fresh-claim test fixture across the suite to pass
  substantive steps; added dedicated _dev_steps_gate coverage.

Commit 2 of 3 for the plan/progress quality work (#171/#172/#173).

tests/integration/v2/test_full_pending_to_completed.py

@@ -62,6 +62,7 @@ class _MockChoreographer:
         _agent_id: object,
         _task_id: object,
         _plan: object = None,
+        **_kwargs: object,
     ) -> Envelope:
         self._state["task_status"] = "in_progress"
         return Envelope.ok(