Add an agents overview dashboard to the TUI (#39094)

## What changed

- Add `/agents` to open a full-screen dashboard of loaded root sessions from the shared app server, with subagent status reflected in each root session.
- Support search, navigation, and grouping sessions by project or status.
- Refresh the dashboard when relevant thread notifications arrive, while preserving its selection and view preferences.
- Show a clear message when the dashboard is unavailable with the embedded app server.

## Testing

- Add coverage for dashboard rendering, status grouping, persisted view state, and restoring the conversation viewport after closing the dashboard.
- Cover `/agents` command discovery and dispatch from parent-owned threads.

GitOrigin-RevId: 6651f4c47358116e1809da3ac8844a5842026a95

codex-rs/tui/src/app/event_dispatch.rs

@@ -2314,6 +2314,12 @@ impl App {
             AppEvent::OpenApprovalsPopup => {
                 self.chat_widget.open_approvals_popup();
             }
+            AppEvent::OpenAgentsOverview => {
+                self.open_agents_overview(app_server);
+            }
+            AppEvent::AgentsOverviewThreadsLoaded { request_id, result } => {
+                self.apply_agents_overview_thread_refresh(app_server, request_id, result);
+            }
             AppEvent::OpenAgentPicker => {
                 self.open_agent_picker(app_server).await;
             }