docs: migrate from MkDocs to Zensical (#144)

[thomaschristory]

Closes #144.

Migrates the docs toolchain from MkDocs + Material for MkDocs to Zensical (the successor, same team). Zensical reads the existing mkdocs.yml natively — no config or content changes, all URLs/anchors preserved.

Changes

• pyproject.toml — drop mkdocs + mkdocs-material, add zensical>=0.0.38 (resolves to 0.0.50), keep pyyaml.
• justfile — just docs → zensical serve, just docs-build → zensical build --strict.
• .github/workflows/docs.yml — build step → uv run zensical build --strict. Output dir stays site/, so the Pages upload path is unchanged. The mkdocs.yml path trigger is kept (still the config file).
• doc refs — release-process.md, gen_docs.py docstring, .gitignore comment updated off "mkdocs".
• mkdocs.yml left untouched per upstream guidance.

Compatibility

Vanilla Material setup — stock theme.features, standard markdown_extensions, no third-party plugins: beyond built-in search — migrates cleanly per the compat matrix.

Verification

• zensical build --strict on a clean tree → No issues found, exit 0.
• Rendered site spot-checked: nav tabs/sections, light/dark palette toggle, admonitions (note/tip/warning), superfences code blocks, code-copy (JS-injected), and search all present.

Note on docs/superpowers/

That dir is gitignored local-only planning docs. MkDocs excluded it via exclude_docs:, which Zensical does not honor — so a local zensical build --strict will warn on its stale cross-links. CI is unaffected (dir is absent in checkouts → clean build). Per maintainer decision, the dir is left in place; move/remove it locally if you want a clean local strict build.

Rollback

mkdocs.yml is untouched — revert is just undoing the deps + command + CI commits.

🤖 Generated with Claude Code