feat(browser-manager): isCustomChromium gate + per-workspace profile + lock cleanup

Three fold-ins so gbrowser can become a thin overlay instead of forking
browse-server:

- Export isCustomChromium(): detects custom Chromium builds that bake the
  extension in as a component extension. Prefers explicit
  GSTACK_CHROMIUM_KIND=custom-extension-baked signal; falls back to
  GSTACK_CHROMIUM_PATH substring containing 'GBrowser' / 'gbrowser'.
  Gates the --load-extension push at launchHeaded so we don't trigger
  ServiceWorkerState::SetWorkerId DCHECK when two copies of the same
  service worker race to register.

- Swap hardcoded path.join(HOME, '.gstack', 'chromium-profile') in
  launchHeaded for resolveChromiumProfile() so phoenix can pass a
  per-workspace profile via CHROMIUM_PROFILE env (one daemon per gbd
  workspace, each with a distinct profile dir).

- Call cleanSingletonLocks(userDataDir) immediately after mkdirSync.
  Chromium's ProcessSingleton refuses to start when stale
  SingletonLock/Socket/Cookie files survive a SIGKILL or hard crash;
  pre-launch cleanup defends against the crash case. Safe under external
  coordination (gbd.lock for gbrowser, single-instance CLI check for
  gstack).

The existing .auth.json write at L291-302 is preserved — extensions
still need it for bootstrap even when component-baked.

Adds browse/test/browser-manager-custom-chromium.test.ts with 8 tests
covering both the env-kind and path-substring signals plus stock /
playwright-bundled Chromium negative cases.

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

browse/test/browser-manager-custom-chromium.test.ts

@@ -0,0 +1,67 @@
+import { describe, test, expect, beforeEach, afterEach } from 'bun:test';
+import { isCustomChromium } from '../src/browser-manager';
+
+describe('browser-manager: isCustomChromium', () => {
+  let origPath: string | undefined;
+  let origKind: string | undefined;
+
+  beforeEach(() => {
+    origPath = process.env.GSTACK_CHROMIUM_PATH;
+    origKind = process.env.GSTACK_CHROMIUM_KIND;
+  });
+
+  afterEach(() => {
+    if (origPath === undefined) delete process.env.GSTACK_CHROMIUM_PATH;
+    else process.env.GSTACK_CHROMIUM_PATH = origPath;
+    if (origKind === undefined) delete process.env.GSTACK_CHROMIUM_KIND;
+    else process.env.GSTACK_CHROMIUM_KIND = origKind;
+  });
+
+  test('GSTACK_CHROMIUM_KIND=custom-extension-baked → true (preferred explicit signal)', () => {
+    delete process.env.GSTACK_CHROMIUM_PATH;
+    process.env.GSTACK_CHROMIUM_KIND = 'custom-extension-baked';
+    expect(isCustomChromium()).toBe(true);
+  });
+
+  test('GSTACK_CHROMIUM_KIND wins even when path is stock Chromium', () => {
+    process.env.GSTACK_CHROMIUM_PATH = '/usr/bin/chromium';
+    process.env.GSTACK_CHROMIUM_KIND = 'custom-extension-baked';
+    expect(isCustomChromium()).toBe(true);
+  });
+
+  test('PascalCase GBrowser in path → true (fallback substring match)', () => {
+    delete process.env.GSTACK_CHROMIUM_KIND;
+    process.env.GSTACK_CHROMIUM_PATH = '/Applications/GBrowser.app/Contents/MacOS/GBrowser';
+    expect(isCustomChromium()).toBe(true);
+  });
+
+  test('lowercase gbrowser in path → true (fallback substring match)', () => {
+    delete process.env.GSTACK_CHROMIUM_KIND;
+    process.env.GSTACK_CHROMIUM_PATH = '/Applications/gbrowser-dev.app/Contents/MacOS/GBrowser';
+    expect(isCustomChromium()).toBe(true);
+  });
+
+  test('both env vars unset → false', () => {
+    delete process.env.GSTACK_CHROMIUM_PATH;
+    delete process.env.GSTACK_CHROMIUM_KIND;
+    expect(isCustomChromium()).toBe(false);
+  });
+
+  test('stock chromium path → false', () => {
+    delete process.env.GSTACK_CHROMIUM_KIND;
+    process.env.GSTACK_CHROMIUM_PATH = '/usr/bin/chromium';
+    expect(isCustomChromium()).toBe(false);
+  });
+
+  test('Playwright bundled chromium path → false', () => {
+    delete process.env.GSTACK_CHROMIUM_KIND;
+    process.env.GSTACK_CHROMIUM_PATH = '/Users/me/Library/Caches/ms-playwright/chromium-1234/chrome-mac/Chromium.app/Contents/MacOS/Chromium';
+    expect(isCustomChromium()).toBe(false);
+  });
+
+  test('GSTACK_CHROMIUM_KIND with unrelated value falls through to path check', () => {
+    process.env.GSTACK_CHROMIUM_KIND = 'something-else';
+    process.env.GSTACK_CHROMIUM_PATH = '/usr/bin/chromium';
+    expect(isCustomChromium()).toBe(false);
+  });
+});