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/slash_command.rs

@@ -1,7 +1,5 @@
-use std::collections::HashMap;
-
 use strum::IntoEnumIterator;
-use strum_macros::AsRefStr; // derive macro
+use strum_macros::AsRefStr;
 use strum_macros::EnumIter;
 use strum_macros::EnumString;
 use strum_macros::IntoStaticStr;
@@ -12,9 +10,12 @@ use strum_macros::IntoStaticStr;
 )]
 #[strum(serialize_all = "kebab-case")]
 pub enum SlashCommand {
+    // DO NOT ALPHA-SORT! Enum order is presentation order in the popup, so
+    // more frequently used commands should be listed first.
     New,
-    ToggleMouseMode,
+    Diff,
     Quit,
+    ToggleMouseMode,
 }
 
 impl SlashCommand {
@@ -26,6 +27,9 @@ impl SlashCommand {
                 "Toggle mouse mode (enable for scrolling, disable for text selection)"
             }
             SlashCommand::Quit => "Exit the application.",
+            SlashCommand::Diff => {
+                "Show git diff of the working directory (including untracked files)"
+            }
         }
     }
 
@@ -36,7 +40,7 @@ impl SlashCommand {
     }
 }
 
-/// Return all built-in commands in a HashMap keyed by their command string.
-pub fn built_in_slash_commands() -> HashMap<&'static str, SlashCommand> {
+/// Return all built-in commands in a Vec paired with their command string.
+pub fn built_in_slash_commands() -> Vec<(&'static str, SlashCommand)> {
     SlashCommand::iter().map(|c| (c.command(), c)).collect()
 }