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/gateway/test_claim_arg_order.py

@@ -260,7 +260,21 @@ async def test_i_will_plan_calls_claim_and_start_with_task_id_first() -> None:
     deps = _make_deps(task=task_svc)
     c = Choreographer(deps)
 
-    env = await c.i_will_plan(pm_id, task_id, plan="break the work into 3 subtasks")
+    env = await c.i_will_plan(
+        pm_id,
+        task_id,
+        plan="break the work into 3 subtasks",
+        rich_plan={
+            "approach": (
+                "Three-cell decomposition: backend, frontend, and ux each "
+                "own a vertical slice of the work."
+            ),
+            "sub_tasks": [
+                {"title": "Backend slice", "description": "API + DB"},
+                {"title": "Frontend slice", "description": "UI integration"},
+            ],
+        },
+    )
 
     task_svc.claim.assert_awaited_once_with(task_id, pm_id)
     task_svc.start.assert_awaited_once_with(task_id, pm_id)