feat: Add symlink-based development workflow for src/resources/ (#744)
- Add scripts/dev-symlink.sh for managing symlinks between src/resources/
and ~/.gsd/agent/
- Supports three modes: create (default), --remove, --status
- Preserves config files: auth.json, models.json, settings.json,
managed-resources.json
- Creates symlinks for: extensions/, skills/, agents/, GSD-WORKFLOW.md
- Backs up existing directories before creating symlinks
- Adds npm scripts: dev:symlink, dev:symlink:remove, dev:symlink:status,
dev:clean, dev:full