swarm: named scheduled prompts for one or more agents, fired in each agent

swarm-controller keeps every scheduled prompt (id, name, agents, body,
anchor, interval, paused) as one document under one key of a new
`schedule-definitions` KV bucket, so each change is a single
compare-and-set and a prompt exists for all of its agents or none. It
writes each agent's key of a new `agent-schedules` bucket as the prompts
naming that agent, without names or other agents, after every change and
on a one-minute reconcile pass. It is the only writer of both, behind
GET/POST /api/schedules and PATCH/DELETE /api/schedules/{id}.
`swarmctl schedule add|list|edit|pause|resume|rm` drives those routes over
the controller socket.

An agent's own queue credential is granted one more subject, the direct
get of its own key in `agent-schedules`. A KV watch is not granted: its
consumer create subject carries no key, so it would cover every agent's.

hive-agent's new schedule ticker reads that key every 10 seconds and fires
due slots as the same `schedule:<id>` todo hive-c0re's worker pushes. Fire
times are anchor + k*interval; the last fired slot per schedule id is kept
in the state db, so a body edit or list refresh never moves or refires a
slot, downtime fires the latest missed slot once, a restart refires
nothing, a one-shot fires once, and paused slots are skipped.

Refs #4914

docs/swarm/README.md

@@ -21,6 +21,8 @@ grafana, the controller, authelia, …) →
 
 - **control plane** — `swarm-controller`: hive directory, agent roster, job
   graph, agent creation. → [below](#swarm-controller)
+- **scheduled prompts** — named prompts for one or more agents at set times,
+  kept by the swarm and fired in each agent's container. → [`schedules.md`](schedules.md)
 - **swarm UI** — the operator's day-to-day surface, on the swarm apex,
   `admins` only. → [`ui.md`](ui.md)
 - **shared services** — one forge, homeserver, SSO, queue and metrics/logs