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/chatwidget/slash_dispatch.rs

@@ -313,6 +313,9 @@ impl ChatWidget {
             SlashCommand::Side | SlashCommand::Btw => {
                 self.request_empty_side_conversation(cmd);
             }
+            SlashCommand::Agents => {
+                self.app_event_tx.send(AppEvent::OpenAgentsOverview);
+            }
             SlashCommand::MultiAgents => {
                 self.app_event_tx.send(AppEvent::OpenAgentPicker);
             }
@@ -1139,6 +1142,7 @@ impl ChatWidget {
             | SlashCommand::Side
             | SlashCommand::Btw
             | SlashCommand::Keymap
+            | SlashCommand::Agents
             | SlashCommand::MultiAgents
             | SlashCommand::Permissions
             | SlashCommand::ElevateSandbox