chore: bump version to 0.6.0

[utensil]

Summary

• Bumps galgebra/_version.py from 0.6.0rc1 to 0.6.0
• Adds :release:0.6.0 <2026.04.01>`` marker to doc/changelog.rst

Release plan (after this PR is merged)

1. Squash merge this PR to master
2. gh release create v0.6.0 --title "v0.6.0" --notes "See [Changelog](https://galgebra.readthedocs.io/en/latest/changelog.html).\n\nhttps://pypi.org/project/galgebra/0.6.0/"
3. CI publishes 0.6.0 to PyPI
4. Zenodo auto-archives (triggers on full releases, not pre-releases)
5. Close 0.6.0 milestone

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.46%. Comparing base (f1288d7) to head (98fbeec).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #562   +/-   ##
=======================================
  Coverage   73.46%   73.46%           
=======================================
  Files          22       22           
  Lines        4459     4459           
=======================================
  Hits         3276     3276           
  Misses       1183     1183           

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