feat(ci): automate release publishing

[Nowhitestar]

Summary

• Add a conservative release planner: feat/perf publishes minor, breaking changes publish major, and release:* can force a bump.
• Add a CI release job after the full test matrix; it prepares versions, runs the release gate/build, atomically pushes main + tag, then publishes the GitHub Release.
• Document the release rule for contributors and maintainers.

Verification

• python -m ruff check .
• python -m ruff format --check .
• python -m pytest -q
• python -m mypy core
• python scripts/check_release.py
• python scripts/test_local.py