tui: render Navi's answers as markdown ...

AssistantMessageRenderer now parses its content as markdown (headings, lists,
bold/italic, inline and fenced code blocks with theme-aware syntax
highlighting) via the existing ThemedMarkdownRenderable — the same renderer
the planning cards already use — instead of plain Text. The formatting
shows live as Navi streams. User messages stay plain text.

Co-Authored-By: Claude <noreply@anthropic.com>