Phase 0: Project Preparation and CI

[wdconinc]

• Add GitHub Actions to build/test C++ and Python wheels for Linux (scikit-build-core, pybind11).
• Add development Dockerfile with all dependencies (libpqxx, Arrow, pybind11, etc).
• Add CONTRIBUTING.md and dev setup docs.
• Add minimal unit test harness (GoogleTest for C++, pytest for Python).
• Standardize DB connection configuration (env var or config file).

Deliverable: CI builds, dev docs, and Docker image.