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_heartbeat_wired.py

@@ -240,6 +240,14 @@ async def test_i_will_plan_calls_heartbeat() -> None:
     deps = _make_deps(task=task_svc)
     c = Choreographer(deps)
 
-    await c.i_will_plan(pm_id, tid, plan="plan-text")
+    await c.i_will_plan(
+        pm_id,
+        tid,
+        plan="plan-text",
+        rich_plan={
+            "approach": "Single-cell decomposition: backend handles all scope.",
+            "sub_tasks": [{"title": "Slice A", "description": "backend API work"}],
+        },
+    )
 
     task_svc.heartbeat.assert_awaited_with(tid)