feat(gateway): restore Gate Set A claim-time guards

Ports five pre-gateway predicates that were dropped when the gateway
displaced the MCP claim handler. Predicates restored from
roboco/mcp/tasks/handlers/_helpers.py:124-204 and
roboco/mcp/tasks/handlers/claim.py:121-180 at commit 254cc93:

- SEQUENCE_ORDER_VIOLATION: a sibling task with sequence < N must be
  in completed/cancelled before sibling N can be claimed.
- ALREADY_ACTIVE: agent cannot claim while owning an in_progress /
  claimed / verifying task other than the one being resumed.
- PAUSED_TASKS_EXIST: agent cannot claim while paused tasks exist.
- PM_CANNOT_EXECUTE_CODE: cell_pm/main_pm cannot claim task_type=code.
- ROLE_TYPED_CLAIM: developer claim is restricted to
  code/research/design; qa/documenter must use claim_review /
  claim_doc_task.

All five guards run inside Choreographer._run_claim_guards before
i_will_work_on / i_will_plan / claim_review / claim_doc_task mutate
state. Skip flags isolate guards that don't apply to a verb (e.g.,
PM-code skipped on QA verb, role-typed skipped on PM verb).

The guards live in roboco/services/gateway/claim_guards.py so
choreographer.py stays focused on orchestration.

Existing tests updated to provide the new mock primings; the
permissive auto-mock behavior they relied on no longer applies.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/gateway/test_choreographer_pm_extras.py

@@ -53,14 +53,34 @@ def _make_deps(**overrides: Any) -> ChoreographerDeps:
 async def test_i_will_plan_claims_starts_and_sets_plan() -> None:
     pm_id = uuid4()
     task_id = uuid4()
-    pending = MagicMock(id=task_id, status="pending", plan=None, assigned_to=None)
-    claimed = MagicMock(id=task_id, status="claimed", plan=None, assigned_to=pm_id)
+    pending = MagicMock(
+        id=task_id,
+        status="pending",
+        plan=None,
+        assigned_to=None,
+        task_type="planning",
+        parent_task_id=None,
+        sequence=0,
+    )
+    claimed = MagicMock(
+        id=task_id,
+        status="claimed",
+        plan=None,
+        assigned_to=pm_id,
+        task_type="planning",
+    )
     started = MagicMock(
-        id=task_id, status="in_progress", plan={"text": "x"}, assigned_to=pm_id
+        id=task_id,
+        status="in_progress",
+        plan={"text": "x"},
+        assigned_to=pm_id,
+        task_type="planning",
     )
     task_svc = AsyncMock()
     task_svc.get.return_value = pending
     task_svc.agent_for.return_value = MagicMock(role="cell_pm", team="backend")
+    task_svc.list_in_progress_for_agent.return_value = []
+    task_svc.list_paused_for_agent.return_value = []
     task_svc.claim.return_value = claimed
     task_svc.set_plan.return_value = claimed
     task_svc.start.return_value = started