feat(sync): GitClient — async wrapper for git CLI

얇은 git CLI 래퍼. F6-L2 sync MVP 의 빌딩 블록.

- run/isRepo/hasRemote/addAll/commit/push/currentBranch
- commit() 은 "nothing to commit" 을 changed=false 로 구분 (정상 path)
- 그 외 실패는 throw, exitCode + stderr 보존
- 8 vitest cases — empty file 로 GIT_CONFIG_GLOBAL/SYSTEM 격리

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main/services/GitClient.ts

@@ -0,0 +1,79 @@
+import { execFile } from 'node:child_process';
+import { promisify } from 'node:util';
+
+const execFileAsync = promisify(execFile);
+
+export interface GitExecResult {
+  stdout: string;
+  stderr: string;
+  exitCode: number;
+}
+
+export interface GitCommitResult {
+  changed: boolean; // false if "nothing to commit"
+  sha: string | null;
+}
+
+export class GitClient {
+  constructor(private cwd: string, private timeoutMs: number = 60_000) {}
+
+  async run(args: string[]): Promise<GitExecResult> {
+    try {
+      const { stdout, stderr } = await execFileAsync(
+        'git',
+        ['-C', this.cwd, ...args],
+        { timeout: this.timeoutMs, maxBuffer: 10 * 1024 * 1024 }
+      );
+      return { stdout, stderr, exitCode: 0 };
+    } catch (e) {
+      const err = e as { stdout?: string; stderr?: string; code?: number };
+      return {
+        stdout: err.stdout ?? '',
+        stderr: err.stderr ?? String(e),
+        exitCode: typeof err.code === 'number' ? err.code : -1
+      };
+    }
+  }
+
+  async isRepo(): Promise<boolean> {
+    const r = await this.run(['rev-parse', '--git-dir']);
+    return r.exitCode === 0;
+  }
+
+  async hasRemote(name: string = 'origin'): Promise<boolean> {
+    const r = await this.run(['remote', 'get-url', name]);
+    return r.exitCode === 0;
+  }
+
+  async addAll(): Promise<void> {
+    const r = await this.run(['add', '-A']);
+    if (r.exitCode !== 0) throw new Error(`git add failed: ${r.stderr}`);
+  }
+
+  async commit(message: string): Promise<GitCommitResult> {
+    // Detect "nothing to commit" via stderr/stdout rather than treating as a thrown error.
+    const r = await this.run(['commit', '-m', message]);
+    if (r.exitCode === 0) {
+      const sha = await this.run(['rev-parse', 'HEAD']);
+      return { changed: true, sha: sha.stdout.trim() };
+    }
+    const blob = (r.stdout + '\n' + r.stderr).toLowerCase();
+    const nothingToCommit = /nothing to commit|nothing added to commit|no changes added to commit/i.test(blob);
+    if (nothingToCommit) {
+      return { changed: false, sha: null };
+    }
+    throw new Error(`git commit failed: ${r.stderr || r.stdout}`);
+  }
+
+  async push(remote: string = 'origin', branch?: string): Promise<void> {
+    const args = branch ? ['push', remote, branch] : ['push', remote];
+    const r = await this.run(args);
+    if (r.exitCode !== 0) throw new Error(`git push failed: ${r.stderr}`);
+  }
+
+  async currentBranch(): Promise<string> {
+    const r = await this.run(['rev-parse', '--abbrev-ref', 'HEAD']);
+    if (r.exitCode !== 0) throw new Error(`git rev-parse failed: ${r.stderr}`);
+    return r.stdout.trim();
+  }
+}