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

Adds `/diff` command to view git diff

codex-cli/src/utils/extract-applied-patches.ts

@@ -0,0 +1,36 @@
+import type { ResponseItem } from "openai/resources/responses/responses.mjs";
+
+/**
+ * Extracts the patch texts of all `apply_patch` tool calls from the given
+ * message history. Returns an empty string when none are found.
+ */
+export function extractAppliedPatches(items: Array<ResponseItem>): string {
+  const patches: Array<string> = [];
+
+  for (const item of items) {
+    if (item.type !== "function_call") {
+      continue;
+    }
+
+    const { name: toolName, arguments: argsString } = item as unknown as {
+      name: unknown;
+      arguments: unknown;
+    };
+
+    if (toolName !== "apply_patch" || typeof argsString !== "string") {
+      continue;
+    }
+
+    try {
+      const args = JSON.parse(argsString) as { patch?: string };
+      if (typeof args.patch === "string" && args.patch.length > 0) {
+        patches.push(args.patch.trim());
+      }
+    } catch {
+      // Ignore malformed JSON – we never want to crash the overlay.
+      continue;
+    }
+  }
+
+  return patches.join("\n\n");
+}