feat: add remote slug helper and auto-gitignore for .gstack/

- getRemoteSlug() in config.ts: parses git remote origin → owner-repo format
- browse/bin/remote-slug: shell helper for SKILL.md use (BSD sed compatible)
- ensureStateDir() now appends .gstack/ to project .gitignore if not present
- setup creates ~/.gstack/projects/ global state directory
- 7 new tests: 4 gitignore behavior + 3 remote slug parsing