feat(kg): add markdown export format for knowledge graph

- Add kg_ocr/export/markdown_exporter.py with export_markdown()
- Cluster entity co-occurrences using Louvain community detection
- Wire -f markdown into `ocr-pipeline kg export` CLI command with KG_REPORT.md default
- Add unit and CLI tests in tests/test_graph.py
- Document markdown export in README.md
- Untrack pycache binaries and ignore graphify-out and KG_REPORT.md

kg_ocr/__init__.py

@@ -25,6 +25,7 @@ _LAZY: dict[str, tuple[str, str]] = {
     "expand_context": (".graph", "expand_context"),
     "export_json": (".export", "export_json"),
     "export_graphml": (".export", "export_graphml"),
+    "export_markdown": (".export", "export_markdown"),
     "load_json": (".export", "load_json"),
     "Neo4jExporter": (".export", "Neo4jExporter"),
 }