ci: pick the docs-preview toolchain from the PR checkout (#3081)

Author: maxisbey

.github/workflows/docs-preview.yml

@@ -1,13 +1,14 @@
 name: Docs Preview
 
-# Builds the mkdocs site for a PR and deploys it to Cloudflare Pages.
+# Builds the docs site for a PR and deploys it to Cloudflare Pages.
 #
-# Security: mkdocs executes Python from the PR (mkdocstrings imports src/mcp,
-# `!!python/name:` directives). The build is gated by `authorize` (admin sender
-# for auto-preview, admin/maintainer commenter for /preview-docs) and isolated
-# from Cloudflare secrets — `build` runs PR code with no secrets and hands the
-# static site to `deploy` via an artifact, so PR code never shares a runner
-# with the Cloudflare token.
+# Security: the build executes Python from the PR (mkdocstrings imports
+# src/mcp, `!!python/name:` config directives run, and heads may ship their
+# own build scripts). The build is gated by `authorize` (admin sender for
+# auto-preview, admin/maintainer commenter for /preview-docs) and isolated
+# from Cloudflare secrets — `build` runs PR code with no secrets and hands
+# the static site to `deploy` via an artifact, so PR code never shares a
+# runner with the Cloudflare token.
 #
 # Required configuration:
 #   - secrets.CLOUDFLARE_API_TOKEN   (scope: Account → Cloudflare Pages → Edit)
@@ -21,6 +22,7 @@ on:
       - docs/**
       - docs_src/**
       - mkdocs.yml
+      - scripts/docs/**
       - pyproject.toml
   issue_comment:
     types: [created]
@@ -128,17 +130,30 @@ jobs:
           enable-cache: false
           version: 0.9.5
 
-      - run: uv sync --frozen --group docs
-      - run: uv run --frozen --no-sync mkdocs build
-        env:
-          # Silence mkdocs-material's MkDocs 2.0 warning banner in CI logs.
-          NO_MKDOCS_2_WARNING: "1"
+      # pull_request_target runs this workflow file from the base branch, so
+      # the whole recipe — dependency sync included — must come from the
+      # checkout itself: heads that ship scripts/docs/build.sh (the Zensical
+      # toolchain) build with it; older heads, and v1.x heads previewed via
+      # /preview-docs, still build with MkDocs. Both arms must write the site
+      # to site/. Keep the detection in sync with build_site() in
+      # scripts/build-docs.sh.
+      - run: |
+          if [ -f scripts/docs/build.sh ]; then
+            bash scripts/docs/build.sh
+          else
+            uv sync --frozen --group docs
+            # The env var silences mkdocs-material's MkDocs 2.0 warning banner.
+            NO_MKDOCS_2_WARNING=1 uv run --frozen --no-sync mkdocs build
+          fi
 
       - uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
           name: site
           path: site/
           retention-days: 1
+          # An empty site/ means the build arm broke its output contract; fail
+          # here instead of surfacing as a confusing download error in deploy.
+          if-no-files-found: error
 
   deploy:
     needs: [authorize, build]