Install @tiptap/react + extensions (starter-kit, link, table,
code-block-lowlight, placeholder) and build KnowledgeEditor component
with full toolbar: headings, bold, italic, strikethrough, lists,
blockquote, code block with syntax highlighting, links, tables,
and horizontal rules. All styles use CSS variables for theme compat.
Implements TW-EDT-001.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>