feat: add team sync infrastructure (config, auth, push/pull, CLI)

- lib/sync-config.ts: reads .gstack-sync.json + ~/.gstack/auth.json
- lib/auth.ts: device auth flow (browser OAuth, local HTTP callback)
- lib/sync.ts: Supabase push/pull via raw fetch(), offline queue, cache
- lib/cli-sync.ts: CLI handler for gstack-sync commands
- bin/gstack-sync: bash wrapper (setup, status, push-*, pull, drain)
- .gstack-sync.json.example: template for team setup

Zero new dependencies — uses raw fetch() against PostgREST API.
All sync is non-fatal with 5s timeout and offline queue fallback.

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

.gstack-sync.json.example

@@ -0,0 +1,5 @@
+{
+  "supabase_url": "https://YOUR_PROJECT.supabase.co",
+  "supabase_anon_key": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.YOUR_ANON_KEY_HERE",
+  "team_slug": "your-team-name"
+}