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.rs

@@ -195,6 +195,13 @@ pub(crate) enum TranscriptExportDestination {
 #[allow(clippy::large_enum_variant)]
 #[derive(Debug)]
 pub(crate) enum AppEvent {
+    /// Open the daemon-wide overview of loaded root sessions and their subagents.
+    OpenAgentsOverview,
+    /// Update the daemon-wide overview after a background thread listing finishes.
+    AgentsOverviewThreadsLoaded {
+        request_id: Uuid,
+        result: Result<Vec<Thread>, String>,
+    },
     /// Open the agent picker for switching active threads.
     OpenAgentPicker,
     /// Merge a completed root-scoped agent-picker refresh without blocking terminal input.