feat(gateway): structured required_cells gate — reject i_am_idle on a dropped named cell

The companion to the prompt rule (60de3499): when the brief explicitly names
cells, the Main PM must create a subtask for each and not silently collapse one
into a neighbour. Records the named cells as a 'required_cells:' marker on the
parent's quick_context (no migration — same pattern as the other markers), and
adds a _pm_uncovered_required_cells_guard at i_am_idle that refuses to idle
while a named cell has no subtask. Inert when no parent carries the marker, so
legacy decompositions are never blocked (mirrors the AC-coverage guard).
TaskService.uncovered_required_cells + extract_required_cells + 7 unit tests.

tests/unit/services/test_required_cells.py

@@ -0,0 +1,74 @@
+"""required_cells decomposition gate — marker parse + uncovered-cell coverage.
+
+The Main PM must create a subtask for each cell the brief explicitly names
+(recorded as a ``required_cells:`` marker on the parent's quick_context). The
+gate is inert when no marker is present, so legacy decompositions never block.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import AsyncMock, MagicMock
+from uuid import uuid4
+
+import pytest
+from roboco.services.task import TaskService, extract_required_cells
+
+# ---------------------------------------------------------------------------
+# extract_required_cells (marker parser)
+# ---------------------------------------------------------------------------
+
+
+def test_extract_required_cells_absent_is_empty() -> None:
+    assert extract_required_cells(None) == []
+    assert extract_required_cells("original_developer: abc\ndoc_notes: y") == []
+
+
+def test_extract_required_cells_parses_and_normalizes() -> None:
+    qc = "original_developer: abc\nrequired_cells: Backend, Frontend , UX/UI"
+    assert extract_required_cells(qc) == ["backend", "frontend", "ux_ui"]
+
+
+def test_extract_required_cells_dedups_in_order() -> None:
+    out = extract_required_cells("required_cells: backend, backend, frontend")
+    assert out == ["backend", "frontend"]
+
+
+# ---------------------------------------------------------------------------
+# uncovered_required_cells (service coverage check)
+# ---------------------------------------------------------------------------
+
+
+def _service(parent_qc: str | None, child_teams: list[str | None]) -> TaskService:
+    """A TaskService whose get()/get_subtasks() return a parent + these children."""
+    svc = TaskService(MagicMock())
+    parent = MagicMock(quick_context=parent_qc)
+    children = [MagicMock(team=t) for t in child_teams]
+    object.__setattr__(svc, "get", AsyncMock(return_value=parent))
+    object.__setattr__(svc, "get_subtasks", AsyncMock(return_value=children))
+    return svc
+
+
+@pytest.mark.asyncio
+async def test_uncovered_inert_without_marker() -> None:
+    svc = _service("doc_notes: x", ["backend"])
+    assert await svc.uncovered_required_cells(uuid4()) == []
+
+
+@pytest.mark.asyncio
+async def test_uncovered_flags_the_dropped_cell() -> None:
+    # Brief named backend+frontend+ux_ui; only backend+frontend got subtasks.
+    svc = _service("required_cells: backend, frontend, ux_ui", ["backend", "frontend"])
+    assert await svc.uncovered_required_cells(uuid4()) == ["ux_ui"]
+
+
+@pytest.mark.asyncio
+async def test_uncovered_empty_when_all_named_cells_covered() -> None:
+    svc = _service("required_cells: backend, frontend", ["frontend", "backend"])
+    assert await svc.uncovered_required_cells(uuid4()) == []
+
+
+@pytest.mark.asyncio
+async def test_uncovered_normalizes_child_team_form() -> None:
+    # Marker uses underscore, child team uses the slash form — they match.
+    svc = _service("required_cells: ux_ui", ["UX/UI"])
+    assert await svc.uncovered_required_cells(uuid4()) == []