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/product_owner.md

@@ -16,6 +16,7 @@
 | Tool | Body schema |
 |------|-------------|
 | `note` | `note(text: str, scope: str = 'note', task_id: UUID | None = None, title: str | None = None, context: str = '', options: list[str | str] | None = None, chosen: str = '', rationale: str = '', consequences: list[str] | None = None, what_done: str = '', what_learned: str = '', what_struggled: str = '', next_steps: list[str] | None = None)` |
+| `pitch` | `pitch(title: str, slug: str, problem: str, proposed_solution: str, target_cells: list[str])` |
 | `say` | `say(channel: str, text: str, task_id: UUID | None = None)` |
 | `dm` | `dm(recipient: str, text: str, task_id: UUID | None = None, skill: str | None = None)` |
 | `notify` | `notify(target: str, text: str, priority: str = 'normal', task_id: UUID | None = None)` |
@@ -24,4 +25,5 @@
 | `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()` |