Write a release procedure runbook (doc/dev/release-process.md)

[utensil]

The 0.6.0 cycle surfaced several non-obvious steps that aren't documented anywhere. This issue tracks writing a doc/dev/release-process.md that a maintainer can follow end-to-end.

Related prior art: #517, v0.5.2rc1 release, PRs #561 and #562.

What the doc should cover:

Policy

• Always cut an RC first to validate the full pipeline before the final release
• RC goes to PyPI as a pre-release; Zenodo skips pre-releases

Branch and version bump

• Create a branch (e.g. utensil/X.Y.Z-rc1), bump _version.py to X.Y.ZrcN (PEP 440) — the version must exactly match the tag that will be pushed (see the 0.5.1 incident in Release 0.5.2 #517)
• After RC validates, create a second branch for the final bump: X.Y.ZrcN → X.Y.Z

Changelog

• The :release: marker in doc/changelog.rst uses semantic_version internally — PEP 440 RC suffixes like 0.6.0rc1 are invalid semver and will break the RTD build. Omit the marker for RCs; add it only for the final release.
• test/.nbval_sanitize.cfg version regex must match all suffix patterns (e.g. rc\d+|-dev) — validate this before tagging

Creating the release

• RC: gh release create vX.Y.ZrcN --prerelease --title "vX.Y.ZrcN" --notes "..."
• Final: gh release create vX.Y.Z --title "vX.Y.Z" --notes "..."
• The CI Create release and send to PyPI job triggers on the tag automatically

Post-release checks

• PyPI: verify at https://pypi.org/project/galgebra/#history
• Zenodo: auto-archives on full releases but the webhook is unreliable — verify and trigger manually from zenodo settings if needed
• Close the milestone
• Update README and user-facing docs referencing version/install instructions (separate PR)