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>

tests/api/test_artifacts.py

@@ -356,3 +356,16 @@ class ArtifactAPITestCase(APITestCase):
             )
             self.assertEqual(response.status_code, 400, field)
             self.assertIn(field, response.data["detail"])
+
+    # -- syntax highlighting -------------------------------------------------
+
+    def test_markdown_fenced_code_block_gets_syntax_highlighted(self):
+        response = self.create_artifact(
+            content="# Test\n\n```python\ndef hello():\n    return \"world\"\n```"
+        )
+        artifact = Artifact.objects.get(pk=response.data["id"])
+        stored_html = (self.artifacts_root / artifact.path).read_text(encoding="utf-8")
+
+        self.assertIn('class="codehilite"', stored_html)
+        self.assertIn('<span class="k">def</span>', stored_html)
+        self.assertIn('href="http://testserver/static/themes/pygments.css"', stored_html)