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/gateway/test_claim_arg_order.py

@@ -275,11 +275,26 @@ async def test_i_will_plan_calls_claim_and_start_with_task_id_first() -> None:
         rich_plan={
             "approach": (
                 "Three-cell decomposition: backend, frontend, and ux each "
-                "own a vertical slice of the work."
+                "own a vertical slice of the work. Backend lands first, QA "
+                "reviews each PR after it opens, documentation follows, then "
+                "complete and submit up. Strict sequencing with no cross-cell "
+                "dependencies beyond the stated ordering."
             ),
             "sub_tasks": [
-                {"title": "Backend slice", "description": "API + DB"},
-                {"title": "Frontend slice", "description": "UI integration"},
+                {
+                    "title": "Backend slice",
+                    "description": (
+                        "be-dev-1 implements the API + DB migration with "
+                        "tests and opens the leaf PR for QA review."
+                    ),
+                },
+                {
+                    "title": "Frontend slice",
+                    "description": (
+                        "fe-dev-1 wires the UI integration with loading and "
+                        "error states and opens the leaf PR for QA."
+                    ),
+                },
             ],
         },
     )