Add git status utility for TUI.

Provides GitStatus interface and refreshGitStatus function for displaying
repository branch, dirty state, untracked files, and commit counts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/sf-tui/git.ts

@@ -0,0 +1,70 @@
+import { execFileSync } from "node:child_process";
+
+export interface GitStatus {
+  branch: string | null;
+  dirty: boolean;
+  untracked: boolean;
+  ahead: number;
+  behind: number;
+}
+
+let cache: GitStatus | null = null;
+let lastFetch = 0;
+
+export function refreshGitStatus(cwd: string): GitStatus {
+  const now = Date.now();
+  if (now - lastFetch < 400 && cache) return cache;
+  lastFetch = now;
+
+  let branch: string | null = null;
+  try {
+    branch = execFileSync("git", ["branch", "--show-current"], {
+      cwd,
+      encoding: "utf-8",
+      stdio: ["pipe", "pipe", "ignore"],
+      timeout: 1500,
+    }).trim() || null;
+  } catch {
+    cache = { branch: null, dirty: false, untracked: false, ahead: 0, behind: 0 };
+    return cache;
+  }
+
+  try {
+    const status = execFileSync("git", ["status", "--porcelain"], {
+      cwd,
+      encoding: "utf-8",
+      stdio: ["pipe", "pipe", "ignore"],
+      timeout: 1500,
+    });
+    const lines = status.split("\n").filter((l) => l.length > 2);
+    const dirty = lines.some((l) => {
+      const x = l[0] ?? " ";
+      const y = l[1] ?? " ";
+      return x !== "?" && x !== " " && x !== "!" || y !== " " && y !== "?";
+    });
+    const untracked = lines.some((l) => l.startsWith("??"));
+
+    let ahead = 0;
+    let behind = 0;
+    try {
+      const ab = execFileSync("git", ["rev-list", "--left-right", "--count", "HEAD...@{u}"], {
+        cwd,
+        encoding: "utf-8",
+        stdio: ["pipe", "pipe", "ignore"],
+        timeout: 1500,
+      }).trim();
+      const [a, b] = ab.split("\t").map((n) => parseInt(n, 10));
+      ahead = Number.isNaN(a) ? 0 : a;
+      behind = Number.isNaN(b) ? 0 : b;
+    } catch { /* no upstream */ }
+
+    cache = { branch, dirty, untracked, ahead, behind };
+  } catch {
+    cache = { branch, dirty: false, untracked: false, ahead: 0, behind: 0 };
+  }
+  return cache;
+}
+
+export function invalidateGitStatus(): void {
+  lastFetch = 0;
+}