fix: add file-lock fallback for single-instance guard

Co-authored-by: Haze <hazeone@users.noreply.github.com>

tests/unit/process-instance-lock.test.ts

@@ -0,0 +1,80 @@
+import { existsSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { afterEach, describe, expect, it } from 'vitest';
+import { acquireProcessInstanceFileLock } from '@electron/main/process-instance-lock';
+
+const tempDirs: string[] = [];
+
+function createTempDir(): string {
+  const dir = mkdtempSync(join(tmpdir(), 'clawx-instance-lock-'));
+  tempDirs.push(dir);
+  return dir;
+}
+
+afterEach(() => {
+  while (tempDirs.length > 0) {
+    const dir = tempDirs.pop();
+    if (!dir) continue;
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+describe('process instance file lock', () => {
+  it('acquires lock and writes owner pid', () => {
+    const userDataDir = createTempDir();
+    const lock = acquireProcessInstanceFileLock({
+      userDataDir,
+      lockName: 'clawx',
+      pid: 12345,
+    });
+
+    const lockPath = join(userDataDir, 'clawx.instance.lock');
+    expect(lock.acquired).toBe(true);
+    expect(existsSync(lockPath)).toBe(true);
+    expect(readFileSync(lockPath, 'utf8')).toBe('12345');
+
+    lock.release();
+    expect(existsSync(lockPath)).toBe(false);
+  });
+
+  it('rejects a second lock when owner pid is alive', () => {
+    const userDataDir = createTempDir();
+    const first = acquireProcessInstanceFileLock({
+      userDataDir,
+      lockName: 'clawx',
+      pid: 2222,
+      isPidAlive: () => true,
+    });
+
+    const second = acquireProcessInstanceFileLock({
+      userDataDir,
+      lockName: 'clawx',
+      pid: 3333,
+      isPidAlive: () => true,
+    });
+
+    expect(first.acquired).toBe(true);
+    expect(second.acquired).toBe(false);
+    expect(second.ownerPid).toBe(2222);
+
+    first.release();
+  });
+
+  it('replaces stale lock file when owner pid is not alive', () => {
+    const userDataDir = createTempDir();
+    const lockPath = join(userDataDir, 'clawx.instance.lock');
+    writeFileSync(lockPath, '4444', 'utf8');
+
+    const lock = acquireProcessInstanceFileLock({
+      userDataDir,
+      lockName: 'clawx',
+      pid: 5555,
+      isPidAlive: () => false,
+    });
+
+    expect(lock.acquired).toBe(true);
+    expect(readFileSync(lockPath, 'utf8')).toBe('5555');
+    lock.release();
+  });
+});