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.
Renn F