fix(ai-markmap-agent): preserve YAML frontmatter in translation cleanup

- Fix clean_translated_content() to preserve opening --- for YAML frontmatter
- Auto-add --- if content starts with YAML key (e.g., title:) but missing ---
- Fix MD file: add missing opening ---
- Fix HTML: generate to correct directory (docs/pages/mindmaps/)
- Delete wrongly placed HTML from docs/mindmaps/

Author: lufftw