feat(#2290): split ensure_runtime — dirs to lifecycle, listeners to mcp_sockets supervisor

- lifecycle::ensure_agent_runtime_dir(name): pure filesystem op, no
  Coordinator dep. Creates /run/hyperhive/agents/<name> without touching
  the MCP listener map.

- workers/mcp_sockets::spawn_poll(coord): 10 s reconcile loop (same shape
  as agent_sockets::spawn_poll). Converges 'agent running => MCP listener
  bound'. First tick is immediate so hive-c0re restarts re-register all
  running agents without waiting a full interval. Fixes the dead-listener-
  after-daemon-restart gap.

- All ensure_runtime() call sites updated:
  - Prebuild/Swap/WriteDropin: Coordinator::agent_dir() (pure, no IO)
  - Reconcile-Start: ensure_agent_runtime_dir + agent_dir (dir may be
    missing after reboot; listener deferred to supervisor)
  - run_create / handle_spawn: ensure_agent_runtime_dir + register_agent
    (eager on first spawn so socket ready before harness first turn)
  - apply_commit / merge_config_pr: ensure_agent_runtime_dir + agent_dir
  - Manager (auto_update): ensure_agent_runtime_dir + agent_dir
    (manager has no MCP listener; socket_server::start_manager owns it)

- ensure_runtime() retained in Coordinator with updated doc pointing at
  the preferred split form. No callers remain outside tests.

hive-c0re/src/server.rs

@@ -202,7 +202,12 @@ async fn dispatch(req: &HostRequest, coord: Arc<Coordinator>) -> HostResponse {
 /// registration and notifying the manager on failure.
 async fn handle_spawn(coord: &Arc<Coordinator>, name: &str) -> Result<HostResponse> {
     tracing::info!(%name, "spawn");
-    let agent_dir = coord.ensure_runtime(name)?;
+    // Create the bind-mount source dir (pure filesystem, no Coordinator dep).
+    // MCP listener registration happens eagerly here (not deferred to the
+    // supervisor) so the socket is ready before the harness's first turn.
+    lifecycle::ensure_agent_runtime_dir(name)?;
+    coord.register_agent(name)?;
+    let agent_dir = Coordinator::agent_dir(name);
     let hive = coord.hive_env();
     let paths = Coordinator::agent_paths(name, agent_dir);
     match lifecycle::spawn(name, &hive, &paths).await {