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/codex_delegate.rs

@@ -52,6 +52,7 @@ pub(crate) async fn run_codex_conversation_interactive(
         Arc::clone(&parent_session.services.skills_manager),
         initial_history.unwrap_or(InitialHistory::New),
         SessionSource::SubAgent(SubAgentSource::Review),
+        parent_session.services.agent_control.clone(),
     )
     .await?;
     let codex = Arc::new(codex);