feat(gateway): decomposition coverage gate + AC visibility (guardrails spec 2)

The decomposition floor that pairs with the roll-up gate (spec 4): a PM
cannot finish decomposing a parent while one of its acceptance criteria has
no subtask responsible for it — the "two leaves, half the ACs silently
dropped" pattern. Three parts:

- Gate: i_am_idle is rejected for a cell_pm/main_pm whose owned parent still
  has criteria in unclaimed_parent_acceptance_criteria (claimed = referenced
  by any live, non-cancelled child). Distinct from the roll-up gate, which
  fires at submit_up/complete and demands a *completed* child; this fires
  earlier and asks only that every criterion be *claimed*. Safe-by-
  construction: inert until a PM declares coverage, so legacy / not-yet-
  adopted decompositions are never blocked.

- Visibility: PM-facing briefings (give_me_work, i_will_plan, submit_up) and
  every delegate response now carry parent_ac_coverage ({id,text,claimed,
  verified} per criterion) + unclaimed_parent_acs, so a PM can map subtasks
  to criterion ids via covers_parent_criteria and see what is still
  uncovered after each delegate. Off for leaf roles, so a developer's own
  criteria never surface as bogus "unclaimed" noise.

- Prompts: cell_pm / main_pm role prompts document covers_parent_criteria and
  the new idle enforcement in the existing Coverage discipline.

TaskService.{parent_ac_coverage,unclaimed_parent_acceptance_criteria} added
beside uncovered_parent_acceptance_criteria; all three refactored onto a
shared _parent_ac_ref_sets helper (keeps each under the xenon B ceiling,
preserves the committed roll-up behavior). Verb tables regenerated for the
new delegate param — the regen also syncs pre-existing table drift that was
never regenerated after earlier merges (read_messages, pass_review
ac_verdicts, board pitch). Two brand-new generated tables (prompter,
secretary) are left untracked pending a separate decision.

agents/prompts/_generated/cell_pm.md

@@ -8,7 +8,7 @@
 | Verb | Body schema |
 |------|-------------|
 | `complete` | `complete(task_id: UUID, notes: str)` |
-| `delegate` | `delegate(parent_task_id: UUID, title: str, description: str, assigned_to: str, team: str, task_type: str, nature: str, estimated_complexity: str, acceptance_criteria: list[str], project_id: UUID | None = None)` |
+| `delegate` | `delegate(parent_task_id: UUID, title: str, description: str, assigned_to: str, team: str, task_type: str, nature: str, estimated_complexity: str, acceptance_criteria: list[str], project_id: UUID | None = None, covers_parent_criteria: list[str] | None = None)` |
 | `escalate_up` | `escalate_up(task_id: UUID, reason: str)` |
 | `give_me_work` | `give_me_work()` |
 | `i_am_idle` | `i_am_idle()` |
@@ -35,4 +35,5 @@
 | `notify_list` | `notify_list(unread_only: bool = True, pending_ack_only: bool = False, limit: int = 20)` |
 | `notify_get` | `notify_get(notification_id: UUID)` |
 | `notify_ack` | `notify_ack(notification_id: UUID)` |
+| `read_messages` | `read_messages()` |
 | `channels` | `channels()` |