fix(core): reject explicit execution of a composite task (#958)

Flagged in PR review — the last gap in the "no execution path filters on
is_leaf" class this issue named.

The bulk selectors now filter is_leaf, but they are all *status-based*.
`cf work start <id> --execute` and the v2 execute route with an explicit
task_ids list both call start_task_run directly with an ID, bypassing every one
of those filters, and would walk a composite BACKLOG -> READY -> IN_PROGRESS
and hand it to an engine.

Guarded at start_task_run rather than at each caller: it is the chokepoint both
explicit paths funnel through, and the error names the fix ("run its child
tasks instead") rather than just refusing.

Author: frankbria

codeframe/core/runtime.py

@@ -76,14 +76,26 @@ def start_task_run(workspace: Workspace, task_id: str) -> Run:
         Created Run
 
     Raises:
-        ValueError: If task not found
+        ValueError: If task not found, or if it is a composite (#958)
         InvalidTransitionError: If task can't transition to IN_PROGRESS
     """
     # Get the task
     task = tasks.get(workspace, task_id)
     if not task:
         raise ValueError(f"Task not found: {task_id}")
 
+    # Composites are containers, not work (#958). The bulk selectors filter
+    # is_leaf, but this is the *explicit* path — `cf work start <id>` and the
+    # v2 execute route with an explicit task_ids list both land here, so
+    # without this guard a composite's ID still reaches an engine. Rejecting at
+    # this chokepoint covers every explicit caller at once.
+    if not task.is_leaf:
+        raise ValueError(
+            f"Task {task_id} is a composite (a container for subtasks), not "
+            "executable work. Run its child tasks instead — its status is "
+            "rolled up from theirs."
+        )
+
     # Check if there's already an active run
     active = get_active_run(workspace, task_id)
     if active:

tests/core/test_inert_orchestration_958.py

@@ -265,6 +265,30 @@ def test_an_idle_composite_does_not_block_new_assignments(self, tmp_path):
         assert result.executing_count == 0
         assert result.can_assign, result.reason
 
+    def test_starting_a_composite_explicitly_is_rejected(self, tmp_path):
+        """Bulk selectors filter is_leaf; the explicit path needs its own guard.
+
+        `cf work start <composite-id>` and the v2 execute route with an
+        explicit task_ids list both reach start_task_run directly, bypassing
+        every status-based filter.
+        """
+        from codeframe.core import runtime, tasks
+        from codeframe.core.tasks import TaskStatus
+        from codeframe.core.workspace import create_or_load_workspace
+
+        ws = create_or_load_workspace(tmp_path)
+        parent, child, _ = self._tree(ws)
+
+        with pytest.raises(ValueError, match="composite"):
+            runtime.start_task_run(ws, parent.id)
+
+        # No run was created, and the task was not dragged toward IN_PROGRESS.
+        assert runtime.get_active_run(ws, parent.id) is None
+        assert tasks.get(ws, parent.id).status == TaskStatus.BACKLOG
+
+        # A leaf still starts normally.
+        assert runtime.start_task_run(ws, child.id) is not None
+
     def test_get_ready_task_ids_is_not_capped_at_100(self, tmp_path):
         """The v2 API's run-all-ready route must not silently run a subset."""
         from codeframe.core import runtime, tasks