feat(megatask): per-cell project map root-subtasks (multi-project, multi-cell)

A MegaTask root-subtask can now target an ad-hoc per-cell project map — a
third targeting shape that mirrors the existing product fan-out root. In
RoboCo a project is per-cell (ProjectTable.assigned_cell); a monorepo is N
per-cell projects sharing one git_url. So 'multi-cell' IS 'multi-project',
and a task may mix per-cell projects across products or include OSS-library
projects not in any product.

Storage: migration 052 adds task_cell_projects (mirrors product_projects;
unique per (task, team)). TaskTable gains a cascade-delete cell_projects
relationship; TaskCreateRequest / TaskCreate / Task response carry the map.

Policy: batch.is_branchless_coordination + is_valid_batch_shape gain a
has_cell_projects param — a root-subtask targets exactly one of project /
product / cell-map; the umbrella still targets none. TaskService passes
has_cell_projects at every predicate call site and persists the rows in
create(). _ensure_branch_for_task cuts feature/main_pm/{root} per distinct
project in the map (via _distinct_projects_for_task); _require_target_or_umbrella
and _validate_batch_membership accept the map shape.

Fan-out: every distinct_project_ids site (task.py branch creation, routes
_project_for_complete + _resolve_project_for_merge, orchestrator
_ambient_projects_for_task, pr_review._project_slug_for, git._project_for_task)
generalizes to first-distinct-project-of-map-or-product. Choreographer
_resolve_subtask_project resolves a delegated subtask's cell from the parent's
cell map. The product-scoped _slugs_for_product intake helper is unchanged.

Intake: prompter._draft_cell_map extracts the per-cell map from the_work[].
_validate_batch_scope counts distinct projects across all drafts' cells
(>=2 min stays; one 2-cell draft satisfies it). create_task_from_draft
persists cell_projects for >=2-cell drafts (project_id/product_id None),
collapses a 1-cell map to the single-project shape, and leaves single-cell
top-level project_id drafts unchanged. _resolve_owning_team routes a
multi-cell map to Main PM (coordination root, like a product root — a cell
PM can't delegate cross-cell). propose_draft/propose_batch tool descriptions
declare the per-cell project_id (both Claude SDK + grok runtimes).

The umbrella stays branchless / pure-coordination / submit_root-rejected;
the CEO-escalation pr_number gate is not widened (the map root is
is_umbrella=False, mirroring a product root, so submit_root supplies it).
Single-cell root-subtasks and everything below them are byte-for-byte
unchanged. Un-run MegaTask waves (multi-cell drafts) become runnable.

tests/unit/api/test_schemas_tasks.py

@@ -30,6 +30,7 @@ from roboco.api.schemas.tasks import (
     transform_update_data,
 )
 from roboco.models.base import Complexity, TaskNature, TaskStatus, TaskType, Team
+from roboco.models.product import ProductCellMapping
 
 _ORDER_DEFAULT = 0
 
@@ -291,6 +292,7 @@ def _stub_task(*, with_project: bool = False) -> SimpleNamespace:
         task_type=TaskType.CODE,
         project_id=uuid4(),
         product_id=None,
+        cell_projects=[],
         project=(SimpleNamespace(slug="proj-1") if with_project else None),
         docs_complete=False,
         pr_created=False,
@@ -348,6 +350,38 @@ def test_task_to_response_includes_slug_when_project_loaded() -> None:
     assert resp.project_slug == "proj-1"
 
 
+def test_task_to_response_serializes_cell_projects_when_loaded() -> None:
+    """An ad-hoc per-cell map (a multi-cell MegaTask root-subtask) round-trips
+    into the response when the relationship is loaded."""
+    be_proj, fe_proj = uuid4(), uuid4()
+    stub = _stub_task()
+    stub.project_id = None
+    stub.product_id = None
+    stub.cell_projects = [
+        SimpleNamespace(team=Team.BACKEND, project_id=be_proj),
+        SimpleNamespace(team=Team.FRONTEND, project_id=fe_proj),
+    ]
+    fake_inspector = MagicMock()
+    fake_inspector.unloaded = set()  # cell_projects IS loaded
+    with patch("roboco.api.schemas.tasks.sa_inspect", return_value=fake_inspector):
+        resp = task_to_response(stub)  # type: ignore[arg-type]
+    assert resp.cell_projects == [
+        ProductCellMapping(team=Team.BACKEND, project_id=be_proj),
+        ProductCellMapping(team=Team.FRONTEND, project_id=fe_proj),
+    ]
+
+
+def test_task_to_response_omits_cell_projects_when_unloaded() -> None:
+    """A freshly-created task whose cell_projects relationship is unloaded
+    serializes to [] rather than triggering a lazy load."""
+    stub = _stub_task()
+    fake_inspector = MagicMock()
+    fake_inspector.unloaded = {"cell_projects"}
+    with patch("roboco.api.schemas.tasks.sa_inspect", return_value=fake_inspector):
+        resp = task_to_response(stub)  # type: ignore[arg-type]
+    assert resp.cell_projects == []
+
+
 def test_task_to_response_serializes_all_note_sections() -> None:
     """Regression: pr_reviewer_notes / doc_notes / notes_structured MUST be in the
     response. The builder previously omitted them, so the panel showed them blank