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/common/test_codex.rs

@@ -143,6 +143,7 @@ impl TestCodexBuilder {
             config.model_provider.clone(),
             config.codex_home.clone(),
         );
+        let conversation_manager = Arc::new(conversation_manager);
 
         let new_conversation = match resume_from {
             Some(path) => {
@@ -164,7 +165,7 @@ impl TestCodexBuilder {
             config,
             codex: new_conversation.conversation,
             session_configured: new_conversation.session_configured,
-            conversation_manager: Arc::new(conversation_manager),
+            conversation_manager,
         })
     }