Add a dedicated codex agents dashboard command (#39114)

## What changed

- Add `codex agents` to open the shared agents overview without creating a new session.
- Start the local background app server automatically on Unix, or connect to a server supplied with `--remote`.
- Reject invocation-specific session overrides that cannot apply to shared sessions.
- When the overview is opened from an embedded session, offer to start the background server without moving or interrupting the current session.

## Testing

- Cover command-line parsing and rejection of incompatible overrides.
- Snapshot the embedded-session background-server prompt.

GitOrigin-RevId: 60845dfebc48d820dc4ff090626d30452127f062

codex-rs/tui/src/app_event.rs

@@ -220,6 +220,14 @@ pub(crate) enum AppEvent {
     StopAgentsOverviewThread {
         thread_id: ThreadId,
     },
+    /// Start the shared app-server daemon without moving the current embedded session.
+    #[cfg(unix)]
+    StartAgentsDaemon,
+    /// Report whether starting the shared app-server daemon succeeded.
+    #[cfg(unix)]
+    AgentsDaemonStarted {
+        result: Result<(), String>,
+    },
     /// Open the agent picker for switching active threads.
     OpenAgentPicker,
     /// Merge a completed root-scoped agent-picker refresh without blocking terminal input.