CI/CD pipeline

[aaronsb]

Set up GitHub Actions for automated testing and publishing.

• Unit tests on push (make test — Python + Rust + C++)
• Publish Python to PyPI on tag
• Publish Rust to crates.io on tag
• Build release binaries (bmapctl for x86_64/aarch64)

[aaronsb]

First checkbox done: .github/workflows/test.yml runs make python-test, rust-test, cpp-test on ubuntu and macOS for pushes to main and all PRs. First run: https://github.com/aaronsb/bosectl/actions/runs/32602913764 (both green). Publishing to PyPI/crates.io and release binaries remain — those are #7/#8 territory and need secrets.