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>

scripts/build-site.sh

@@ -0,0 +1,87 @@
+#!/usr/bin/env sh
+# Build the full multi-version site into ./_site/:
+#   - "latest" from the current HEAD
+#   - one subtree per v* git tag
+#
+# Each per-version build gets MDBOOK_OUTPUT__HTML__SITE_URL set so its
+# canonical URL and sitemap are correct for the deployed location.
+# Emits _site/versions.json (newest first; consumed by
+# theme/version-picker.js) and a root index.html that redirects to
+# /latest/.
+#
+# Intended to be called via `just build-site`. SITE_BASE can be
+# overridden to deploy under a different URL prefix; the default
+# targets https://swaits.github.io/meshcore-spec/.
+
+set -eu
+
+SITE_BASE="${SITE_BASE:-/meshcore-spec}"
+WORKSPACE="$(pwd)"
+SITE_DIR="$WORKSPACE/_site"
+
+# Section grouping for GitHub Actions logs; noop locally.
+group() { printf '::group::%s\n' "$1"; }
+endgroup() { printf '::endgroup::\n'; }
+warn() { printf '::warning::%s\n' "$1"; }
+
+rm -rf "$SITE_DIR"
+mkdir -p "$SITE_DIR"
+
+# ---- latest (current HEAD) ----
+group "build latest"
+MDBOOK_OUTPUT__HTML__SITE_URL="${SITE_BASE}/latest/" \
+    mdbook build "$WORKSPACE" -d "$SITE_DIR/latest"
+endgroup
+
+# ---- each v* tag ----
+# Each tag is built from its own git worktree so today's mdbook config
+# / theme files don't bleed into older builds.
+TAGS="$(git tag --list 'v*' --sort=-v:refname 2>/dev/null || true)"
+for tag in $TAGS; do
+    [ -z "$tag" ] && continue
+    group "build $tag"
+    wt="$(mktemp -d -t mdbook-tag-XXXXXX)"
+    git worktree add --detach "$wt" "$tag"
+    if [ -f "$wt/book.toml" ]; then
+        if ! MDBOOK_OUTPUT__HTML__SITE_URL="${SITE_BASE}/${tag}/" \
+             mdbook build "$wt" -d "$SITE_DIR/${tag}"; then
+            warn "mdbook build failed for ${tag}; skipping"
+            rm -rf "${SITE_DIR:?}/${tag:?}"
+        fi
+    else
+        warn "${tag} has no book.toml; skipping"
+    fi
+    git worktree remove --force "$wt"
+    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 "$SITE_DIR/$tag" ] || continue
+        printf ',"%s"' "$tag"
+    done
+    printf ']\n'
+} > "$SITE_DIR/versions.json"
+echo "versions.json:"
+cat "$SITE_DIR/versions.json"
+
+# ---- root redirect to /latest/ ----
+cat > "$SITE_DIR/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