Enables codehilite on the markdown renderer (Pygments, 'friendly' style,
vendored as static/themes/pygments.css) so fenced code blocks get real
syntax highlighting with zero client-side JS - matching the "themes are
CSS-only" stance in framework_artifact_rendering.md. Verified live against
the running dev instance: a fenced python block renders with highlighted
keyword/string/function spans.
This was Gap 2 from planning/2026-09-02-artifact_kind_gaps_for_frequent_agent_output.md,
picked as the first fix since it needed no new kind or design decision -
just enabling an existing markdown extension.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
