Build and deploy via GitHub Actions

[zyansheep]

Replaces the local-build + second-repo publishing setup with a GitHub Actions deploy.

Why it was the old way

All on 2022-04-18, in four commits: a CI workflow was added (1824e2d), then the build dir was converted to a submodule (8ffe8c5), then the workflow was removed (7fac135). The workflow it gave up on couldn't have worked:

• # extended: true was commented out, but the hugo-book theme compiles SCSS and needs Hugo extended
• it only ran hugo, never mdbook — the whole /docs/ half of the site would have been missing
• actions/deploy-pages didn't exist yet, so the only option was pushing to a branch, which conflicts with public/ being a submodule

None of those still apply. Pages on this repo is already set to build_type: "workflow".

What changes

• .github/workflows/deploy.yml — builds on push to main and on PRs, deploys only from main. Pins hugo-extended 0.115.0 / mdbook 0.4.31 / mdbook-katex 0.5.4 to match the nix devshell, installed from prebuilt binaries (~10s, no cargo install).
• build.sh — one build path shared by the devshell and CI. Clears public/ first, which drops stale fingerprinted assets that had piled up because Hugo never removes old output.
• Drops the public submodule and gitignores the directory. libdither/dither.link-build (7.6 MB of committed build output vs 1.4 MB of source) stops being written to.
• Removes build-push.sh; fixes the flake buildPhase, which called hugoix and had therefore never worked.

Two things that are load-bearing and easy to break later:

• submodules: recursive is required — /docs/ is built from dither-spec, and static/disp/interactive-walkthrough.html is a symlink into the disp submodule that Hugo follows.
• dither-spec/book.toml sets src = ".", so mdbook copies every non-markdown file in the submodule into /docs/. Locally that swept in .git and a nested public/docs/ tree left by running mdbook there without -d — the latter is on the live site today. build.sh strips both so local builds match a fresh CI checkout.

Verification

The CI Pages artifact was downloaded and diffed against a local ./build.sh: identical, 190 files, with /docs/, the symlinked disp page, CNAME and 404.html all present.

Against the currently-published site the only differences are removals: two orphaned search-index files from an older build, and the nested docs/public/ tree. Nothing links to either.

After merge

The first deploy lands at https://libdither.github.io/dither.link/. The dither.link custom domain still points at dither.link-build until it's moved over — DNS is unchanged, but GitHub re-provisions the TLS cert, which takes a few minutes.

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Investigated, written and verified by Claude (Claude Code) on behalf of @zyansheep. The code, commit messages and this description were all drafted by Claude; commits carry a Co-Authored-By trailer.