feat: add support for /diff command (#1389)

Adds support for a `/diff` command comparable to the one available in
the TypeScript CLI.

<img width="1103" alt="Screenshot 2025-06-26 at 12 31 33 PM"
src="https://github.com/user-attachments/assets/5dc646ca-301f-41ff-92a7-595c68db64b6"
/>

While here, changed the `SlashCommand` enum so the declared variant
order is the order the commands appear in the popup menu. This way,
`/toggle-mouse-mode` is listed last, as it is the least likely to be
used.

Fixes https://github.com/openai/codex/issues/1253.

codex-rs/tui/src/app.rs

@@ -1,6 +1,7 @@
 use crate::app_event::AppEvent;
 use crate::app_event_sender::AppEventSender;
 use crate::chatwidget::ChatWidget;
+use crate::get_git_diff::get_git_diff;
 use crate::git_warning_screen::GitWarningOutcome;
 use crate::git_warning_screen::GitWarningScreen;
 use crate::login_screen::LoginScreen;
@@ -250,6 +251,27 @@ impl<'a> App<'a> {
                     SlashCommand::Quit => {
                         break;
                     }
+                    SlashCommand::Diff => {
+                        let (is_git_repo, diff_text) = match get_git_diff() {
+                            Ok(v) => v,
+                            Err(e) => {
+                                let msg = format!("Failed to compute diff: {e}");
+                                if let AppState::Chat { widget } = &mut self.app_state {
+                                    widget.add_diff_output(msg);
+                                }
+                                continue;
+                            }
+                        };
+
+                        if let AppState::Chat { widget } = &mut self.app_state {
+                            let text = if is_git_repo {
+                                diff_text
+                            } else {
+                                "`/diff` — _not inside a git repository_".to_string()
+                            };
+                            widget.add_diff_output(text);
+                        }
+                    }
                 },
             }
         }