docs-publish fails building stable docs: privacy enabled must be a boolean #9011

Issue by Claude Code, at @rfay's request.

Summary

docs-publish.yml has failed on main since #9009, in the "Build stable docs" step, for example run 40035392301:

Error: privacy enabled must be a boolean

#9009 fixed this for zensical 0.0.72 on main by changing mkdocs.yml to enabled: !ENV [ READTHEDOCS, false ]. The stable branch (v1.25.4) still has enabled: !ENV [ READTHEDOCS, '' ], and the GitHub Pages workflow builds stable too.

Why the PR passed

• docs-check.yml builds only the PR's checkout, so it never builds stable.
• docs-publish.yml installs main's docs/requirements.txt (zensical>=0.0.72,<1) and then stable's (zensical>=0.0.43,<1) into the same Python environment. Both builds then run on zensical 0.0.72, which rejects stable's '' default.
• ReadTheDocs is not affected. It sets READTHEDOCS, so the !ENV default is never used. Only the GitHub Pages build, where READTHEDOCS is unset, fails.

This will clear up by itself at the next release, when docs-stable.yml resets stable to the new tag. Until then, every push to main fails docs-publish.

Suggested fixes

1. Apply the feat(docs): add social cards and llms.txt, fix zensical 0.0.72 privacy error #9009 mkdocs.yml change to stable now, as described in Updating Stable Docs Without a Release:

   git fetch upstream
   git checkout -b stable upstream/stable
   # mkdocs.yml: enabled: !ENV [ READTHEDOCS, false ]
   git commit -am "docs: make privacy.enabled a boolean for zensical 0.0.72 [skip ci]"
   git push upstream stable

   [skip ci] also skips docs-publish.yml for that push, so the GitHub Pages site is rebuilt on the next push to main. ReadTheDocs built the same line for feat(docs): add social cards and llms.txt, fix zensical 0.0.72 privacy error #9009 without problems.

2. Build stable with its own dependencies in docs-publish.yml. Install each checkout's docs/requirements.txt into its own virtualenv, and pin zensical exactly in docs/requirements.txt. Then a zensical change on main can't break the stable build, and stable keeps building with the version it was released and tested with.

3. Wait for v1.25.5,, probably Monday which fixes this when that's the new stable

Fix 1 unblocks the workflow today. Fix 2 keeps the next zensical breaking change from failing the same way.