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_main_pm.py

@@ -208,12 +208,27 @@ async def test_i_will_plan_dispatches_to_choreographer() -> None:
             "task_id": _TASK_ID,
             "plan": "split into backend, frontend, ux cells",
             "approach": (
-                "Three-cell decomposition: backend handles API, frontend "
-                "handles UI integration, ux-ui handles design."
+                "Three-cell decomposition: backend handles the API, frontend "
+                "handles UI integration, ux-ui handles design. Sequenced so "
+                "backend lands first, QA reviews each PR after it opens, "
+                "documentation follows, then complete and submit up. No "
+                "cross-cell dependencies beyond the stated ordering."
             ),
             "sub_tasks": [
-                {"title": "Backend cell", "description": "API implementation"},
-                {"title": "Frontend cell", "description": "UI integration"},
+                {
+                    "title": "Backend cell",
+                    "description": (
+                        "be-dev-1 implements the API endpoint and migration "
+                        "with tests, opens the leaf PR for QA."
+                    ),
+                },
+                {
+                    "title": "Frontend cell",
+                    "description": (
+                        "fe-dev-1 wires the panel to the new endpoint with "
+                        "loading and error states, opens the leaf PR."
+                    ),
+                },
             ],
         },
         headers=_HEADERS,