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/startup_draft.rs

@@ -183,7 +183,9 @@ impl StartupDraftPump {
         session_selection: &SessionSelection,
     ) -> io::Result<()> {
         let session_action = match session_selection {
-            SessionSelection::StartFresh | SessionSelection::Exit => StartupDraftSessionAction::New,
+            SessionSelection::StartFresh
+            | SessionSelection::Exit
+            | SessionSelection::AgentsOverview => StartupDraftSessionAction::New,
             SessionSelection::Resume(_) => StartupDraftSessionAction::Resume,
             SessionSelection::Fork(_) => StartupDraftSessionAction::Fork,
         };