Add syntax highlighting for fenced code blocks in markdown artifacts

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>

artifacts/views.py

@@ -502,7 +502,16 @@ def create_text_artifact(data, request, user):
     if kind == Artifact.HTML_FULL:
         rendered = content
     elif kind == Artifact.MARKDOWN:
-        rendered = wrap_html(markdown.markdown(content, extensions=["extra", "sane_lists"]), title, theme, request)
+        rendered = wrap_html(
+            markdown.markdown(
+                content,
+                extensions=["extra", "sane_lists", "codehilite"],
+                extension_configs={"codehilite": {"guess_lang": False}},
+            ),
+            title,
+            theme,
+            request,
+        )
         (target_dir / f"{artifact_id}.md").write_text(content, encoding="utf-8")
     elif kind == Artifact.DECK:
         rendered = wrap_deck(content, title, request)
@@ -595,6 +604,7 @@ def create_binary_artifact(data, user):
 
 def wrap_html(body, title, theme, request):
     stylesheet = request.build_absolute_uri(f"/static/themes/{theme}.css")
+    pygments_stylesheet = request.build_absolute_uri("/static/themes/pygments.css")
     return f"""<!doctype html>
 <html>
 <head>
@@ -602,6 +612,7 @@ def wrap_html(body, title, theme, request):
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>{html.escape(title)}</title>
   <link rel="stylesheet" href="{stylesheet}">
+  <link rel="stylesheet" href="{pygments_stylesheet}">
 </head>
 <body class="artifact">
 {body}