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,

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,

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

@@ -19,7 +19,19 @@ _AGENT_ID = "00000000-0000-0000-0004-000000000001"
 _HEADERS = {"X-Agent-ID": _AGENT_ID, "X-Agent-Role": "main_pm"}
 _HTTP_UNPROCESSABLE = 422
 _HTTP_OK = 200
-_MIN_APPROACH_LEN = 20
+# #171: raised 20→150 — must stay in sync with IWillPlanRequest.approach
+# min_length and choreographer._impl._PM_APPROACH_MIN_LEN.
+_MIN_APPROACH_LEN = 150
+_GOOD_APPROACH = (
+    "Single-cell decomposition for the git-workflow smoke test: be-pm owns "
+    "the backend slice end to end — claim, branch, delegate the README edit "
+    "to be-dev-1, sequence QA after the PR opens, then documentation, then "
+    "complete and submit_up. Frontend and UX cells are unaffected."
+)
+_GOOD_SUBTASK_DESC = (
+    "be-dev-1 creates the feature branch, prepends the smoke-test HTML "
+    "comment above the README H1 leaving the rest untouched, commits, opens PR."
+)
 
 
 def _make_envelope(error: str, missing: list[str] | None = None) -> MagicMock:
@@ -76,10 +88,7 @@ def test_i_will_plan_rejects_empty_subtasks_for_pm() -> None:
         json={
             "task_id": str(uuid4()),
             "plan": "Route to backend cell only",
-            "approach": (
-                "Single-cell decomposition: backend cell handles the "
-                "smoke test end-to-end; frontend and ux unaffected."
-            ),
+            "approach": _GOOD_APPROACH,
             "sub_tasks": [],  # empty — gateway rejects for PM
         },
     )
@@ -101,11 +110,8 @@ def test_i_will_plan_schema_accepts_rich_plan() -> None:
     req = IWillPlanRequest(
         task_id=uuid4(),
         plan="Route to backend",
-        approach=(
-            "Single-cell decomposition for the smoke test: be-pm handles "
-            "git workflow validation end to end."
-        ),
-        sub_tasks=[{"title": "Backend slice", "description": "Branch + edit + PR"}],
+        approach=_GOOD_APPROACH,
+        sub_tasks=[{"title": "Backend slice", "description": _GOOD_SUBTASK_DESC}],
         risks=[],
         open_questions=[],
     )