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 locations and exclude pyspark
  from xdist parallel runs to prevent fork OOM on CI runners
- Implemented path-based filtering using `dorny/paths-filter@v3` to
  replicate CircleCI's conditional job execution
- Limited Spark driver memory to 512m in tests to prevent OOM
- Configured engine-specific environment variables and auth for all cloud
  and Docker test targets

Signed-off-by: eakmanrq <6326532+eakmanrq@users.noreply.github.com>

Author: eakmanrq