Migrate docs site from MkDocs/Material to Zensical #10

Summary

Replace MkDocs + Material for MkDocs with Zensical as the generator for docs.freeshard.net.

Blocked by #4 (move the blog to the Astro landing page). Zensical does not support the Material blog plugin yet (zensical/backlog#30, open, no activity since 2025-11-13). Once #4 lands and the blog plugin is dropped from mkdocs.yml, this repo has zero unsupported plugins and the migration is unblocked — no need to wait on backlog#30.

Why

• MkDocs 1.x is unmaintained (no release in ~2 years).
• MkDocs 2.0 is not an upgrade path: no plugin system, TOML config, no migration path, and explicitly incompatible with Material for MkDocs.
• Material for MkDocs has been in maintenance mode since 2025-11-06, with 12 months of critical bug/security fixes guaranteed (so through roughly November 2026).
• Zensical is the successor by the same author, MIT-licensed, and reads mkdocs.yml natively.

Not urgent — this is a static site generator, so an unpatched build chain is not a live attack surface. But the stack should not stay frozen indefinitely.

Compatibility check (done 2026-07-25)

Trial-built this repo with uvx --with markdown-include zensical build (Zensical 0.0.51) against a scratch copy and diffed the HTML against the MkDocs output:

Side effect worth keeping: Zensical reports missing anchors as build warnings. It found a broken link in a 2023 blog post that mkdocs build --strict never flagged (MkDocs only logs those at INFO). Fixed in #9.

Sequencing

Order matters. Doing the swap before the blog has moved would silently break the blog: Zensical renders posts at /blog/posts/<dir>/main/ instead of Material's /blog/YYYY/MM/DD/<slug>/, and the archive index comes out empty.

1. Move blog onto freeshard.net via Astro content collections (from docs MkDocs) #4 ships: blog live on freeshard.net, 301 redirects in place.
2. Remove blog from the plugins: list in mkdocs.yml, delete docs/blog/.
3. This issue: swap the generator.

Scope

• Re-run the trial build against the post-Move blog onto freeshard.net via Astro content collections (from docs MkDocs) #4 tree; diff the generated HTML against the MkDocs output page by page.
• Visual pass: Mermaid diagrams, dark mode, code blocks with line numbers and annotations, glightbox image zoom.
• Replace mkdocs/mkdocs-material in requirements.txt with zensical (keep markdown-include; mkdocs-glightbox can go, Zensical handles it natively).
• Swap the build command in .github/workflows/ci.yml (mkdocs build --strict → zensical build) and confirm it still emits to public/.
• Keep mkdocs.yml as-is initially — Zensical reads it. Converting to zensical.toml is optional and can wait for the announced migration tool.
• Update agents.md (tech stack, commands, deployment).

Risks

• Zensical is 0.0.x alpha and iterating fast — pin the version in requirements.txt.
• No multi-version docs support yet. We do not use versioning, so this does not affect us.
• Theme overrides would need rethinking under Zensical's theming model. We have none active (custom_dir is commented out; docs/css/extra.css is small), so this is cheap for us.