Add empirical weight calibration for scoring policy

- Implemented `WeightCalibrator` for fitting OLS regression to map score breakdowns to reference scores.
- Introduced `calibrate_from_processed_artifacts` for convenience in reading existing artifacts and running calibration.
- Added detailed docstrings and usage examples in `calibration.py`.
- Created unit tests for calibration logic, including OLS helpers, weight normalization, and edge cases.
- Developed tests for Markdown export functionality, ensuring proper formatting and content generation.

Author: MatALass

src/portfolio_auditor/exports/__init__.py

@@ -1,7 +1,9 @@
 from portfolio_auditor.exports.csv_exporter import CsvExporter
 from portfolio_auditor.exports.json_exporter import JsonExporter
+from portfolio_auditor.exports.markdown_exporter import MarkdownExporter
 
 __all__ = [
     "CsvExporter",
     "JsonExporter",
-]
+    "MarkdownExporter",
+]