hive-c0re: remove the approvals subsystem

Nothing submits an approval since scheduled prompts left hive-c0re, so the
queue only acted on rows written before that.

- hive-c0re: the approvals store and its migrations, approve/deny/
  resolve actions, the ResolveApproval tail node (and meta_update's
  approval_id), POST /api/approve and /api/deny, the approval views on
  /api/state, the approval_resolved dashboard event, the orphan GC,
  destroy/purge failing pending rows, and the approval rows in
  get_loose_ends. DestroyBookkeeping loses the purge flag, which only
  worded the approval-failure note.
- The CancelLooseEnd request goes: its only hive-c0re arm was the
  approval one. cancel_loose_end handles reminder and todo in the harness.
- Wire types: hive_sh4re::approvals (ReminderStats moves to inbox),
  LooseEnd::Approval, CancelLooseEndKind, HelperEvent::ApprovalResolved,
  ToolGroup::Approvals, HostRequest::{Pending,Approve,Deny} and
  HostResponse.approvals. HOST_SOCK_VERSION goes to 2 for the removed
  field.
- hivectl loses the approvals verbs; docs/tools/hivectl-cli.md is
  regenerated.
- The system prompt drops approval_resolved.
- The approvals table stays in broker.sqlite, unread, the same way the
  operator_questions table was left when its store was removed.
- An `approvals` entry still in tool-groups.json is skipped with a warning
  by ToolGroup::parse_list, like any unknown group.
- docs/agent-lifecycle/approvals.md is renamed to config-changes.md so the
  code's doc pointers resolve; its content follows in the docs commit.

swarm-controller/src/config_pr.rs

@@ -119,9 +119,8 @@ const POLL_INTERVAL: std::time::Duration = std::time::Duration::from_mins(5);
 /// A full replace rather than an incremental merge: `Client::list_open_config_prs`
 /// already returns the complete current set (an agent with no open PR is
 /// simply absent), so merging would need its own stale-entry eviction to
-/// avoid an agent's long-closed PR lingering forever — the same reconcile
-/// problem `hive-c0re`'s poller solves for its approvals. A full replace
-/// sidesteps needing that logic twice: the map at any moment is nothing more
+/// avoid an agent's long-closed PR lingering forever. A full replace
+/// sidesteps needing that logic: the map at any moment is nothing more
 /// than "the last successful scan's answer."
 pub struct ConfigPrCache(Mutex<HashMap<String, ConfigPrStatus>>);