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/dashboard/state_snapshot.rs

@@ -152,8 +152,7 @@ struct ApprovalView {
     #[serde(skip_serializing_if = "Option::is_none")]
     description: Option<String>,
     /// Raw `commit_ref` payload for `UpdateMetaInputs` (JSON-encoded
-    /// `Vec<String>` of input names; `"[]"` = all inputs) and
-    /// `SchedulePrompt` (JSON-encoded `SchedulePromptPayload`). The
+    /// `Vec<String>` of input names; `"[]"` = all inputs). The
     /// frontend parses this to render a human-readable card body.
     commit_ref: String,
     /// RFC 3339 UTC time the approval was queued. Rendered as a