Fix gh-pages deploy: clean untracked build artifacts before commit

Build artifacts (dist/, internal/, webmanager/) were leaking into
the gh-pages branch because git checkout doesn't remove untracked
files. Add git clean -fd after switching branches.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ale-rinaldi

.github/workflows/build.yml

@@ -43,6 +43,9 @@ jobs:
 
         git checkout gh-pages
 
+        # Clean untracked files left over from the build step
+        git clean -fd
+
         # Remove old builds for this branch, keep others
         rm -rf "$BRANCH"
         mkdir -p "$BRANCH"