feat: add --gstack mode for GStack Desktop integration

[lancepan123]

Summary

Add a new --gstack flag that routes browser commands through GStack Desktop's Unix domain socket IPC instead of discovering a CDP endpoint automatically.

This enables bb-browser CLI to drive an Electron-based desktop app (GStack Desktop) via its IPC interface, while preserving full CDP fallback for unsupported operations.

Changes

File	Description
packages/cli/src/gstack-bridge.ts	New — Bridge module (execFileSync → gstack CLI → IPC socket). Exports: gstackGetTabs, gstackFindTabByDomain, gstackOpenTab, gstackEvaluate, gstackScreenshot, gstackNavigate
packages/cli/src/index.ts	--gstack flag parsing, handleGstackCommand() intercept layer in main() — supported commands: open, screenshot, tab, eval, back/forward/refresh; unsupported commands fall through to CDP
packages/cli/src/commands/site.ts	GStack execution path in siteRun() — tab discovery by domain → open if needed → evaluate adapter script → auth error detection
packages/cli/src/cdp-client.ts	createTargetWithFallback() — gracefully handles runtimes where Target.createTarget is not supported (e.g. some Electron builds)
packages/cli/src/commands/open.ts	Relax tabId parsing to accept both numeric index and string/UUID IDs

How it works

bb open https://example.com --gstack
#  → gstack CLI → Unix socket (~/.gstack/ipc.sock) → Electron IPC → BrowserWindow

When --gstack is set:

1. handleGstackCommand() intercepts known commands and routes them via gstack-bridge.ts
2. Unknown commands fall through to the normal CDP dispatch path
3. site command uses gstack tab discovery + evaluate instead of CDP Runtime.evaluate

Testing

• Build: pnpm build passes with zero TS errors
• Runtime verified: IPC ping, snapshot, click, fill, evaluate all working
• CDP fallback verified: snapshot, click, fill work normally when --gstack is not set