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

README.md

@@ -79,6 +79,7 @@ uv run ocr-pipeline kg query data/ocr_output/kg_graph.json --entity BRCA1 --expa
 uv run ocr-pipeline kg query data/ocr_output/kg_graph.json --citation 10.1038/nature12345
 
 # export
+uv run ocr-pipeline kg export data/ocr_output/kg_graph.json -f markdown
 uv run ocr-pipeline kg export data/ocr_output/kg_graph.json -f graphml
 uv run ocr-pipeline kg export data/ocr_output/kg_graph.json -f neo4j
 ```
@@ -239,4 +240,3 @@ Gene/protein names go through scispaCy's `en_core_sci_lg`. Chemical formulas
 and units (µM, ng/mL, kb/Mb/Gb, °C, ×g) get normalized, scientific notation
 gets cleaned up (`1.5×10⁻³` → `1.5×10^-3`), and gel/blot figures get their
 captions pulled out separately.
-