feat: /diff command to view git diff (#426)

Adds `/diff` command to view git diff

codex-cli/tests/slash-commands.test.ts

@@ -10,6 +10,7 @@ test("SLASH_COMMANDS includes expected commands", () => {
   expect(commands).toContain("/model");
   expect(commands).toContain("/approval");
   expect(commands).toContain("/clearhistory");
+  expect(commands).toContain("/diff");
 });
 
 test("filters slash commands by prefix", () => {

codex-cli/tests/terminal-chat-input-compact.test.tsx

@@ -17,6 +17,7 @@ describe("TerminalChatInput compact command", () => {
       setItems: () => {},
       contextLeftPercent: 10,
       openOverlay: () => {},
+      openDiffOverlay: () => {},
       openModelOverlay: () => {},
       openApprovalOverlay: () => {},
       openHelpOverlay: () => {},