fix: resolve nullability warnings, add CI/CD and security workflows, harden .gitignore
- Add nullable annotations across discovery interfaces, dynamic query
models, and filter/aggregate types to eliminate CS8600-series warnings
- Replace unsafe cast in DynamicQueryHandlerBase with pattern match
- Add CI workflow (build --warnaserror + test on JP branch)
- Add weekly security vulnerability scan workflow
- Extend .gitignore with secret/credential patterns (.env, *.key, secrets/, credentials.json)
Co-Authored-By: Svrnty Inc. <eng@svrnty.com>
5f3602d071