chore: fetch brand CSS from canonical source at build time

Single source of truth for navi.css lives in the brand repo.
CI fetches it before every build so docs sites stay in sync.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Fieldnote-Echo

.github/workflows/docs.yml

@@ -29,6 +29,10 @@ jobs:
       - name: Install docs dependencies
         # For library repos, remove --no-install-project so the package is importable
         run: uv sync --frozen --group docs
+      - name: Fetch brand CSS
+        run: |
+          curl -sL https://raw.githubusercontent.com/Project-Navi/brand/main/docs/stylesheets/navi.css \
+            -o docs/stylesheets/navi.css
       - name: Build site
         run: uv run zensical build
       - name: Strip internal docs from build