feat(#2290): converge unification cleanup — pull preamble into lifecycle

Collapse the scattered ensure_agent_runtime_dir calls into the lifecycle
functions themselves so callers have a single responsibility:

- lifecycle::spawn: calls ensure_agent_runtime_dir before write_dropins.
  Callers (handle_spawn, ensure_root_agent) no longer need a separate
  preamble step.

- lifecycle::rebuild_no_meta spawn path: calls ensure_agent_runtime_dir
  before write_dropins. apply_commit / merge_config_pr flows no longer
  need a manual ensure_agent_runtime_dir.

- run_create (job-queue): drops ensure_agent_runtime_dir + register_agent.
  The tail Reconcile's converge_start_preamble handles the runtime dir
  and mcp_sockets::spawn_poll handles the listener. Create stays purely
  'provision + create', not 'create + start'.

- handle_spawn (server.rs): drops manual preamble; lifecycle::spawn owns it.
  Drops unneeded unregister_agent on failure (supervisor handles listener).

- ensure_root_agent (auto_update.rs): drops manual ensure_agent_runtime_dir.

- actions.rs apply_commit / merge_config_pr: drop manual
  ensure_agent_runtime_dir; rebuild_no_meta's spawn path handles it.

Result: ensure_agent_runtime_dir lives in exactly two places —
lifecycle::spawn (direct spawn) and converge_start_preamble (start/reconcile
path). All other callers are clean call sites.

hive-c0re/src/workers/auto_update.rs

@@ -153,9 +153,8 @@ pub async fn ensure_root_agent(coord: &Arc<Coordinator>) -> Result<()> {
         return Ok(());
     }
     tracing::info!("manager container missing — spawning");
-    lifecycle::ensure_agent_runtime_dir(MANAGER_NAME)?;
-    // Manager has no MCP listener (socket_server::start_manager owns its
-    // socket); just need the dir + path value.
+    // lifecycle::spawn creates the runtime dir internally; no manual
+    // ensure_agent_runtime_dir needed here.
     let runtime = Coordinator::agent_dir(MANAGER_NAME);
     let hive = coord.hive_env();
     let paths = Coordinator::agent_paths(MANAGER_NAME, runtime);