Migrate CircleCI to GitHub Actions

Replaces CircleCI configuration with native GitHub Actions workflows:

- Deleted `.circleci/config.yml` and `.circleci/continue_config.yml`
- Moved helper scripts from `.circleci/` to `.github/scripts/`
- Added comprehensive `pr.yaml` with all test jobs (doc tests, style checks, Windows tests, migration tests, UI tests, engine tests for 8 local + 8 cloud databases)
- Created `release.yaml` for PyPI publishing and GitHub releases on version tags
- Removed private repo integration (deleted `private-repo-test.yaml`)
- Updated `Makefile` to reference new script location
- Implemented path-based filtering using `dorny/paths-filter@v3` to replicate CircleCI's conditional job execution

All CircleCI jobs are now running in GitHub Actions with proper dependencies, caching, and secret management.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: eakmanrq