ci(mdbook): drive CI through mise + just (no inline shell)

CI now mirrors the local dev path. The workflow installs mise,
trusts the config, installs all mise-pinned tools (mdbook, just),
and invokes `just build-site` for the whole multi-version build.
There is no longer any build logic inside the workflow YAML.

Changes:

- `mise.toml` now also pins `just = "1.50.0"` so `mise install`
  brings in both mdbook and just.
- `scripts/build-site.sh` (new, POSIX sh, shellcheck-clean) takes
  the multi-version build loop out of the workflow and parks it
  next to the rest of the build tooling. Iterates v* tags via
  `git worktree`, sets `MDBOOK_OUTPUT__HTML__SITE_URL` per
  version, emits `_site/versions.json` and `_site/index.html`
  (root redirect to /latest/).
- `justfile` gains a `build-site` recipe — `sh scripts/build-site.sh`.
- `.github/workflows/mdbook.yml` is now four short steps inside
  the build job: jdx/mise-action (install: false) → `mise trust`
  → `mise install` → `just build-site`. The previous inline
  multi-line bash block is gone.
- `.gitignore` adds `/_site/` alongside the existing `/book/`.

Local smoke test: `just build-site` with no v* tags produces
`_site/{latest/, versions.json=["latest"], index.html}` exactly as
CI will. Future `v0.1.0` tag will add `_site/v0.1.0/` and update
`versions.json` without any code changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/mdbook.yml

@@ -18,7 +18,8 @@ concurrency:
   cancel-in-progress: false
 
 env:
-  # Repo path on GitHub Pages — change here if the repo is renamed.
+  # Repo path on GitHub Pages. Forwarded to scripts/build-site.sh so
+  # per-version SITE_URLs are correct. Change here if the repo moves.
   SITE_BASE: /meshcore-spec
 
 jobs:
@@ -30,81 +31,23 @@ jobs:
         with:
           fetch-depth: 0
 
-      - name: Install mise + pinned tools (mdbook, etc.)
+      - name: Install mise
         uses: jdx/mise-action@v2
         with:
-          install: true
+          install: false
           cache: true
 
+      - name: Trust mise config
+        run: mise trust
+
+      - name: Install pinned tools (mdbook, just)
+        run: mise install
+
       - name: Configure Pages
         uses: actions/configure-pages@v5
 
-      - name: Build all versions
-        run: |
-          set -euo pipefail
-
-          mkdir -p _site
-          WORKSPACE="$PWD"
-
-          # ---- latest (current ref) ----
-          echo "::group::build latest"
-          MDBOOK_OUTPUT__HTML__SITE_URL="${SITE_BASE}/latest/" \
-            mdbook build "$WORKSPACE" -d "$WORKSPACE/_site/latest"
-          echo "::endgroup::"
-
-          # ---- each v* tag ----
-          # Tags are built in worktrees so the workflow's own files (this
-          # file, the picker JS/CSS) don't bleed into older builds.
-          mapfile -t TAGS < <(git tag --list 'v*' --sort=-v:refname)
-          for tag in "${TAGS[@]:-}"; do
-            [ -z "$tag" ] && continue
-            echo "::group::build $tag"
-            wt="$(mktemp -d -t "mdbook-${tag//\//-}-XXXXXX")"
-            git worktree add --detach "$wt" "$tag"
-            # If a very old tag lacks book.toml, skip it but don't fail.
-            if [ -f "$wt/book.toml" ]; then
-              MDBOOK_OUTPUT__HTML__SITE_URL="${SITE_BASE}/${tag}/" \
-                mdbook build "$wt" -d "$WORKSPACE/_site/${tag}" || {
-                  echo "::warning::mdbook build failed for ${tag}; skipping"
-                  rm -rf "$WORKSPACE/_site/${tag}"
-                }
-            else
-              echo "::warning::${tag} has no book.toml; skipping"
-            fi
-            git worktree remove --force "$wt"
-            echo "::endgroup::"
-          done
-
-          # ---- versions.json (consumed by theme/version-picker.js) ----
-          # Newest first; "latest" always at the top.
-          {
-            printf '['
-            printf '"latest"'
-            for tag in "${TAGS[@]:-}"; do
-              [ -z "$tag" ] && continue
-              [ -d "_site/${tag}" ] || continue
-              printf ',"%s"' "$tag"
-            done
-            printf ']\n'
-          } > _site/versions.json
-          echo "versions.json:"
-          cat _site/versions.json
-
-          # ---- root redirect to /latest/ ----
-          cat > _site/index.html <<'HTML'
-          <!DOCTYPE html>
-          <html lang="en">
-          <head>
-            <meta charset="utf-8">
-            <meta http-equiv="refresh" content="0; url=./latest/">
-            <link rel="canonical" href="./latest/">
-            <title>MeshCore Protocol Specification</title>
-          </head>
-          <body>
-            <p>Redirecting to the <a href="./latest/">latest version</a>&hellip;</p>
-          </body>
-          </html>
-          HTML
+      - name: Build full multi-version site
+        run: just build-site
 
       - name: Upload Pages artifact
         uses: actions/upload-pages-artifact@v3