feat(gateway): A1 plan-required-at-claim gate

i_will_plan now requires approach (min_length=20) at the schema and
non-empty sub_tasks at the gateway when the caller is a PM role. Restores
pre-gateway parity for _validate_claimed_start — agents could not
transition claimed -> in_progress without filling the rich plan.

Smoke run 3 (2026-05-11) showed PMs calling i_will_plan with just
plan='paragraph' and the gateway accepting it; Plan tab stayed empty
because no agent filled approach/sub_tasks/risks/open_questions.

Spec ref: docs/superpowers/specs/2026-05-12-post-smoke-3-fixes-design.md
section A1.

tests/unit/api/routes/v2/test_flow_main_pm.py

@@ -204,7 +204,18 @@ async def test_i_will_plan_dispatches_to_choreographer() -> None:
 
     resp = client.post(
         "/api/v2/flow/main_pm/i_will_plan",
-        json={"task_id": _TASK_ID, "plan": "split into backend, frontend, ux cells"},
+        json={
+            "task_id": _TASK_ID,
+            "plan": "split into backend, frontend, ux cells",
+            "approach": (
+                "Three-cell decomposition: backend handles API, frontend "
+                "handles UI integration, ux-ui handles design."
+            ),
+            "sub_tasks": [
+                {"title": "Backend cell", "description": "API implementation"},
+                {"title": "Frontend cell", "description": "UI integration"},
+            ],
+        },
         headers=_HEADERS,
     )