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_choreographer_claim_guards.py

@@ -383,7 +383,20 @@ async def test_cell_pm_can_plan_code_typed_parent_via_i_will_plan() -> None:
     deps = _make_deps(task=task_svc)
     c = Choreographer(deps)
 
-    env = await c.i_will_plan(pm_id, task_id, plan="Decompose into 2 dev subtasks.")
+    env = await c.i_will_plan(
+        pm_id,
+        task_id,
+        plan="Decompose into 2 dev subtasks.",
+        rich_plan={
+            "approach": (
+                "Split code-typed parent into two developer-claimable subtasks: "
+                "one for API implementation, one for test coverage."
+            ),
+            "sub_tasks": [
+                {"title": "API subtask", "description": "Implement endpoint"},
+            ],
+        },
+    )
     body = env.as_dict()
     # The PM-cannot-execute-code rejection must NOT fire on i_will_plan.
     assert body.get("error") != "not_authorized", (
@@ -422,7 +435,20 @@ async def test_pm_can_plan_non_code_parent() -> None:
     deps = _make_deps(task=task_svc)
     c = Choreographer(deps)
 
-    env = await c.i_will_plan(pm_id, task_id, plan="break it down")
+    env = await c.i_will_plan(
+        pm_id,
+        task_id,
+        plan="break it down",
+        rich_plan={
+            "approach": (
+                "Single-cell decomposition: backend handles the full scope; "
+                "no frontend or ux work required for this planning task."
+            ),
+            "sub_tasks": [
+                {"title": "Backend planning slice", "description": "Scope and assign"}
+            ],
+        },
+    )
     assert env.error is None