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>