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_choreographer_claim_guards.py

@@ -397,11 +397,20 @@ async def test_cell_pm_can_plan_code_typed_parent_via_i_will_plan() -> None:
         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."
+                "Split the code-typed parent into two developer-claimable "
+                "subtasks: one for API implementation, one for test coverage. "
+                "Sequenced so the API lands first; QA reviews each PR after "
+                "it opens, documentation follows, then complete and submit "
+                "up. No cross-cell dependencies for this slice."
             ),
             "sub_tasks": [
-                {"title": "API subtask", "description": "Implement endpoint"},
+                {
+                    "title": "API subtask",
+                    "description": (
+                        "be-dev-1 implements the endpoint with tests, commits "
+                        "with the task-id prefix, opens the leaf PR for QA."
+                    ),
+                },
             ],
         },
     )
@@ -450,10 +459,19 @@ async def test_pm_can_plan_non_code_parent() -> None:
         rich_plan={
             "approach": (
                 "Single-cell decomposition: backend handles the full scope; "
-                "no frontend or ux work required for this planning task."
+                "no frontend or ux work required for this planning task. "
+                "be-dev-1 owns the change end to end; QA reviews after the "
+                "PR opens, documentation follows, then be-pm completes and "
+                "submits up. Strict sequencing, no cross-cell dependencies."
             ),
             "sub_tasks": [
-                {"title": "Backend planning slice", "description": "Scope and assign"}
+                {
+                    "title": "Backend planning slice",
+                    "description": (
+                        "scope the change, assign be-dev-1, who implements "
+                        "with tests and opens the leaf PR for QA review."
+                    ),
+                }
             ],
         },
     )