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/interaction.rs

@@ -1,6 +1,7 @@
 //! Key routing and composer-adjacent UI interaction for `ChatWidget`.
 
 use super::*;
+use crate::bottom_pane::BottomPaneView;
 
 impl ChatWidget {
     pub(crate) fn keymap_contexts(&self) -> crate::keymap::KeymapContextSet {
@@ -241,6 +242,21 @@ impl ChatWidget {
         self.request_redraw();
     }
 
+    pub(crate) fn show_bottom_pane_view(&mut self, view: Box<dyn BottomPaneView>) {
+        self.bottom_pane.show_view(view);
+        self.refresh_plan_mode_nudge();
+        self.request_redraw();
+    }
+
+    pub(crate) fn replace_bottom_pane_view_if_present(
+        &mut self,
+        view_id: &'static str,
+        view: Box<dyn BottomPaneView>,
+    ) {
+        self.bottom_pane.replace_view_if_present(view_id, view);
+        self.refresh_plan_mode_nudge();
+    }
+
     pub(crate) fn selected_index_for_present_view(&self, view_id: &'static str) -> Option<usize> {
         self.bottom_pane.selected_index_for_present_view(view_id)
     }

codex-rs/tui/src/chatwidget/rendering.rs

@@ -7,6 +7,16 @@ use std::cell::Cell;
 
 impl ChatWidget {
     pub(crate) fn as_renderable(&self) -> RenderableItem<'_> {
+        if self
+            .bottom_pane
+            .selected_index_for_active_view(crate::app::AGENTS_OVERVIEW_VIEW_ID)
+            .is_some()
+        {
+            return self
+                .bottom_pane
+                .as_renderable_with_composer_right_reserve(/*composer_right_reserve*/ 0);
+        }
+
         let active_cell_right_reserve = self.ambient_pet_wrap_reserved_cols();
         let active_cell_renderable = match &self.transcript.active_cell {
             Some(cell) => RenderableItem::Owned(Box::new(TranscriptAreaRenderable {

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