hive: remove the read-only schedules page and the old schedule store

Scheduled prompts live at swarm level (swarm-controller's
/api/schedules, `swarmctl schedule`, the swarm UI's /schedules page).
The hive dashboard's SCH3DUL3S tab only listed the rows left in
hive-c0re's `scheduled_prompts` table so operators could copy them
over; the operator confirmed everything is migrated.

Removed:
- the SCH3DUL3S tab, its pane, schedules.js, its CSS (including the
  `.rqe-source*` rules, whose only emitter was schedules.js) and the
  hub tile's "schedules" entry
- hive-c0re's GET /api/schedules and the `scheduled_prompts` store
  (with its v1 column migration and test)
- hive-sh4re's `schedule` wire types, which only that route used
- docs describing the tab, the route and the two tables

The rebuild-queue cancel route that shared the module moves to
dashboard/rebuild_queue.rs under an OpenAPI tag of its own.

The two sqlite tables are not dropped: nothing opens them any more,
and persistence.md says they can be dropped by hand.

Closes #4916

docs/agent-lifecycle/persistence.md

@@ -38,9 +38,8 @@ schemas, file layouts, and internal migration mechanics.
 
 ### `/var/lib/hyperhive/db/broker.sqlite` (host)
 
-Five tables, all in one file — the message queue, a small key/value
-table, the read-only schedule header/targets split, and the per-agent
-power-intent registry:
+Three tables, all in one file — the message queue, a small key/value
+table, and the per-agent power-intent registry:
 
 - `messages` — every inter-agent / operator-authored message.
   `sender / recipient / body / sent_at / delivered_at / acked_at /
@@ -54,17 +53,6 @@ power-intent registry:
   per-agent store — see [`/harness/` contents
   below](#state-dirs-per-agent) for where reminders (and todos)
   live.
-- `scheduled_prompts` — the scheduled prompts left from before they
-  moved to swarm level ([`swarm/schedules.md`](../swarm/schedules.md)).
-  `owner / body / interval_seconds (NULL = one-shot) /
-  next_fire_at_unix / created_at_unix / source ("operator" or
-  "approval:<id>") / cancelled_at_unix / description / paused_at_unix`.
-  Read-only: the dashboard's SCH3DUL3S tab lists them so the operator
-  can recreate them with `swarmctl schedule`; nothing writes or fires
-  them.
-- `scheduled_prompt_targets` — per-target state for each schedule.
-  `schedule_id / target / cancelled_at_unix /
-  last_fired_at_unix / last_result`.
 - `agent_power` — one row per agent: `agent PK / wanted (up |
   offline) / updated_at`, owned by `hive-c0re/src/stores/power.rs`.
   This is the durable power *intent* the job queue reconciles the
@@ -74,6 +62,12 @@ power-intent registry:
   section](../scheduler/coordinator.md#desired-state-spec-vs-status) for who
   writes and reads it and how reconciliation works.
 
+On a host whose hive-c0re once kept scheduled prompts, the file also
+holds `scheduled_prompts` and `scheduled_prompt_targets`. Nothing opens
+them; scheduled prompts live at swarm level
+([`swarm/schedules.md`](../swarm/schedules.md)). They're safe to drop by
+hand.
+
 Retention:
 
 - `Broker::vacuum_delivered` runs hourly via a tokio task in
@@ -82,7 +76,6 @@ Retention:
   rows are always kept — the harness `ack_turn`s only after a
   successful turn, so `requeue_inflight` can still requeue
   an unacked row on a crash.
-- Scheduled prompts: nothing deletes them; the rows stay as they are.
 - `agent_power` rows live until the operator destroys the agent (one row per
   agent — nothing to vacuum).