hive-c0re stops owning scheduled prompts; agents lose the schedule tools

Scheduled prompts are swarm-level now (swarmctl schedule, #4936). The hive
keeps the rows it has only so the operator can copy them over.

- hive-c0re no longer fires anything: the scheduled-prompts worker is gone,
  and with it every create/edit/cancel/pause/resume/fire path (dashboard
  endpoints, agent-socket verbs, the SchedulePrompt approval kind and its
  approve handler). The store keeps open + list; GET /api/schedules stays.
- A v6 approvals migration drops schedule_prompt rows.
- Nothing submits approvals any more, so the submit path goes too:
  Approvals::submit_kind (now a test fixture), the ApprovalAdded dashboard
  event and its emitter.
- Coordinator::push_todo, whose only caller was the worker, goes, and with
  it hive_host_sock::agent_todo_socket and hive-c0re's hive-agent-sock /
  hive-sock-client deps.
- The schedules_changed dashboard event goes; nothing mutates the rows.
- Agents lose request_schedule_prompt, cancel_schedule, fire_schedule_now,
  edit_schedule and list_schedules, the `scheduling` tool group, their
  system-prompt line and their transcript formatters. `remind` is unchanged.

hive-c0re/src/workers/mod.rs

@@ -1,6 +1,5 @@
-//! Background tasks and periodic sweeps: crash/login watcher, the
-//! scheduled-prompt delivery loop, boot-time auto-update
-//! reconcile, the agent-sockets.json writer loop, the MCP socket listener
+//! Background tasks and periodic sweeps: crash/login watcher, boot-time
+//! auto-update reconcile, the agent-sockets.json writer loop, the MCP socket listener
 //! reconcile loop, knowledge-repo sync, and the swarm wanted-state pull.
 //! Each submodule is re-exported
 //! at the crate root, so `crate::crash_watch::…` etc. keep working unchanged.
@@ -10,5 +9,4 @@ pub mod auto_update;
 pub mod crash_watch;
 pub mod knowledge;
 pub mod mcp_sockets;
-pub mod scheduled_prompts_worker;
 pub mod wanted;