Harden TUI subagent navigation (#39088)

## What changed

- Use `/subagents` consistently for the subagent picker and status UI, removing the `/agent` alias.
- Rejoin already-loaded subagent threads without overriding their existing settings.
- Route notifications and approval requests only to the active thread or its known subagents, discard unrelated startup events, and avoid replaying resolved approvals.

## Testing

- Add coverage for filtering buffered startup events, dropping resolved approvals, preserving owned subagent approvals, and resuming threads without setting overrides.

GitOrigin-RevId: b4e597f7cd4756faccf31d8b964e7a172f5ab0eb

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

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

codex-rs/tui/src/chatwidget/tests/composer_submission.rs

@@ -49,7 +49,7 @@ async fn parent_owned_thread_blocks_all_direct_input_entry_points() {
         "/side inspect this",
         "/archive",
         "/rename",
-        "/agent parent",
+        "/subagents parent",
         "/diff now",
         "!echo blocked",
         " !echo blocked",

codex-rs/tui/src/chatwidget/tests/review_mode.rs

@@ -891,11 +891,11 @@ async fn esc_with_pending_steers_overrides_agent_command_interrupt_behavior() {
     }
 
     chat.bottom_pane
-        .set_composer_text("/agent ".to_string(), Vec::new(), Vec::new());
+        .set_composer_text("/subagents ".to_string(), Vec::new(), Vec::new());
     chat.handle_key_event(KeyEvent::new(KeyCode::Esc, KeyModifiers::NONE));
 
     next_interrupt_op(&mut op_rx);
-    assert_eq!(chat.bottom_pane.composer_text(), "/agent ");
+    assert_eq!(chat.bottom_pane.composer_text(), "/subagents ");
 }
 
 #[tokio::test]