Add tiptap-markdown extension for markdown ↔ rich text conversion. Update KnowledgeEditor to accept/emit markdown content. Replace EntryEditor textarea with KnowledgeEditor (Tiptap WYSIWYG). Update tests for the new editor component. Content flow: markdown (stored) → Tiptap (editing) → markdown (saved). The backend markdown → HTML rendering pipeline remains unchanged. Implements TW-EDT-002. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>