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/slash_command.rs

@@ -40,6 +40,7 @@ pub enum SlashCommand {
     Compact,
     Plan,
     Goal,
+    Agents,
     Side,
     Btw,
     Copy,
@@ -126,6 +127,7 @@ impl SlashCommand {
             SlashCommand::Personality => "choose a communication style for Codex",
             SlashCommand::Plan => "switch to Plan mode",
             SlashCommand::Goal => "set or view the goal for a long-running task",
+            SlashCommand::Agents => "view and switch between all active agent sessions",
             SlashCommand::MultiAgents => "switch between this session's subagents",
             SlashCommand::Side | SlashCommand::Btw => {
                 "start a side conversation in an ephemeral fork"
@@ -187,6 +189,7 @@ impl SlashCommand {
         matches!(
             self,
             SlashCommand::Copy
+                | SlashCommand::Agents
                 | SlashCommand::Export
                 | SlashCommand::Raw
                 | SlashCommand::Diff
@@ -255,7 +258,7 @@ impl SlashCommand {
             | SlashCommand::Btw => true,
             SlashCommand::Rollout => true,
             SlashCommand::TestApproval => true,
-            SlashCommand::MultiAgents => true,
+            SlashCommand::Agents | SlashCommand::MultiAgents => true,
             SlashCommand::Theme | SlashCommand::Pets => false,
         }
     }