feat: agent controller (#8783)

Added an agent control plane that lets sessions spawn or message other
conversations via `AgentControl`.

`AgentBus` (core/src/agent/bus.rs) keeps track of the last known status
of a conversation.

ConversationManager now holds shared state behind an Arc so AgentControl
keeps only a weak back-reference, the goal is just to avoid explicit
cycle reference.

Follow-ups:
* Build a small tool in the TUI to be able to see every agent and send
manual message to each of them
* Handle approval requests in this TUI
* Add tools to spawn/communicate between agents (see related design)
* Define agent types

codex-rs/core/tests/suite/remote_models.rs

@@ -442,7 +442,8 @@ where
 
     mutate_config(&mut config);
 
-    let conversation_manager = Arc::new(ConversationManager::with_models_provider(auth, provider));
+    let conversation_manager = ConversationManager::with_models_provider(auth, provider);
+    let conversation_manager = Arc::new(conversation_manager);
 
     let new_conversation = conversation_manager
         .new_conversation(config.clone())