feat(gateway): substantive-plan gate — approach >=150 + real sub_task descriptions (#171)

Plans were vague because the gate accepted the bare minimum: PM
approach >=20 chars and title-only sub_tasks. minimax wrote exactly
the minimum.

- IWillPlanRequest.approach min_length 20 -> 150 (kept in sync with
  _PM_APPROACH_MIN_LEN; the gate enforces it at the choreographer
  layer too so direct/MCP callers can't bypass the HTTP boundary).
- New _thin_subtask_hint: every PM sub_task must have a title and a
  description >= _PM_SUBTASK_DESC_MIN_LEN (60) saying what the step
  does — each sub_task is both a delegate target AND a
  progress-checklist item, so a title alone is not a plan.
- cell_pm/main_pm role prompts: explicit "the gate REJECTS thin plans"
  framing + concrete sub_task example + the new minimums.
- Updated all affected test fixtures across the suite to use
  substantive approaches/descriptions; added thin-sub_task rejection
  coverage.

Commit 1 of 3 for the plan/progress quality work (#171/#172/#173).

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

@@ -237,11 +237,20 @@ async def test_i_will_plan_dispatches_to_choreographer() -> None:
             "task_id": _TASK_ID,
             "plan": "break into 3 subtasks for backend",
             "approach": (
-                "Decompose into backend API slice, QA verification pass, "
-                "and documentation update."
+                "Decompose into a backend API slice that be-dev-1 owns end "
+                "to end; QA reviews after the PR opens, documentation follows, "
+                "then be-pm completes and submits up. Single-cell — no "
+                "frontend or ux work; strict sequencing with no cross-cell "
+                "dependencies for this planning task."
             ),
             "sub_tasks": [
-                {"title": "Backend API slice", "description": "Implement endpoint"}
+                {
+                    "title": "Backend API slice",
+                    "description": (
+                        "be-dev-1 implements the endpoint with tests, commits "
+                        "with the task-id prefix, opens the leaf PR for QA."
+                    ),
+                }
             ],
         },
         headers=_HEADERS,