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/state/service.rs

@@ -2,6 +2,7 @@ use std::sync::Arc;
 
 use crate::AuthManager;
 use crate::RolloutRecorder;
+use crate::agent::AgentControl;
 use crate::exec_policy::ExecPolicyManager;
 use crate::mcp_connection_manager::McpConnectionManager;
 use crate::models_manager::manager::ModelsManager;
@@ -28,4 +29,5 @@ pub(crate) struct SessionServices {
     pub(crate) otel_manager: OtelManager,
     pub(crate) tool_approvals: Mutex<ApprovalStore>,
     pub(crate) skills_manager: Arc<SkillsManager>,
+    pub(crate) agent_control: AgentControl,
 }