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/src/lib.rs

@@ -15,6 +15,7 @@ pub mod codex;
 mod codex_conversation;
 mod compact_remote;
 pub use codex_conversation::CodexConversation;
+mod agent;
 mod codex_delegate;
 mod command_safety;
 pub mod config;