feat: add git branch name to footer (#589)

packages/cli/src/ui/hooks/useGitBranchName.ts

@@ -0,0 +1,66 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { useState, useEffect, useCallback } from 'react';
+import { exec } from 'node:child_process';
+import fs from 'node:fs';
+import fsPromises from 'node:fs/promises';
+import path from 'path';
+
+export function useGitBranchName(cwd: string): string | undefined {
+  const [branchName, setBranchName] = useState<string | undefined>(undefined);
+
+  const fetchBranchName = useCallback(
+    () =>
+      exec(
+        'git rev-parse --abbrev-ref HEAD',
+        { cwd },
+        (error, stdout, _stderr) => {
+          if (error) {
+            setBranchName(undefined);
+            return;
+          }
+          const branch = stdout.toString().trim();
+          if (branch && branch !== 'HEAD') {
+            setBranchName(branch);
+          } else {
+            setBranchName(undefined);
+          }
+        },
+      ),
+    [cwd, setBranchName],
+  );
+
+  useEffect(() => {
+    fetchBranchName(); // Initial fetch
+
+    const gitHeadPath = path.join(cwd, '.git', 'HEAD');
+    let watcher: fs.FSWatcher | undefined;
+
+    const setupWatcher = async () => {
+      try {
+        await fsPromises.access(gitHeadPath, fs.constants.F_OK);
+        watcher = fs.watch(gitHeadPath, (eventType) => {
+          if (eventType === 'change') {
+            fetchBranchName();
+          }
+        });
+      } catch (_watchError) {
+        // Silently ignore watcher errors (e.g. permissions or file not existing),
+        // similar to how exec errors are handled.
+        // The branch name will simply not update automatically.
+      }
+    };
+
+    setupWatcher();
+
+    return () => {
+      watcher?.close();
+    };
+  }, [cwd, fetchBranchName]);
+
+  return branchName;
+}