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/main.rs

@@ -13,8 +13,8 @@ use hive_sh4re::{HostRequest, HostResponse};
 use hive_c0re::coordinator::{Coordinator, HiveEnv, ServeConfig};
 use hive_c0re::{
     agent_sockets, auto_update, broker, client, crash_watch, dashboard, dashboard_events, forge,
-    job_queue, knowledge, matrix, migrate, reminder_scheduler, scheduled_prompts_worker, server,
-    socket_server,
+    job_queue, knowledge, matrix, migrate, mcp_sockets, reminder_scheduler,
+    scheduled_prompts_worker, server, socket_server,
 };
 
 #[derive(Parser)]
@@ -420,6 +420,13 @@ async fn cmd_serve(
     // is one stat per agent per tick.
     // See `docs/gateway.md::Per-agent unix-socket upstream`.
     agent_sockets::spawn_poll();
+    // MCP socket listener reconcile loop: every 10s re-registers any
+    // running agent that lost its host-side MCP listener (e.g. after a
+    // hive-c0re restart cleared /run/hyperhive/agents/). First tick fires
+    // immediately so restarts re-register all running agents without delay.
+    // Decouples listener registration from the start path — start only needs
+    // lifecycle::ensure_agent_runtime_dir; the supervisor converges the rest.
+    mcp_sockets::spawn_poll(coord.clone());
     // Reminder scheduler: drains due reminders + handles
     // file_path payload persistence. See reminder_scheduler.rs.
     reminder_scheduler::spawn(coord.clone());