feat(converter): implement deterministic JSON to Markdown article converter (spec 005)

tests/fixtures/markdown_conversion/missing_extractor_invalid.json

@@ -0,0 +1,8 @@
+{
+  "crawled_url": "https://example.com/article",
+  "page_title": "Example Title",
+  "trafilatura": {
+    "title": "Example Title",
+    "markdown": "Body text"
+  }
+}