Remove investigation feature completely

- Delete investigation CLI command and related services
- Remove investigation data models and report generators
- Clean up configuration options and documentation
- Update gitignore and remove stale egg-info

Investigation feature is no longer needed and has been fully removed.
Backup preserved in git tag 'pre-investigation-removal'.

config.test.yaml

@@ -72,7 +72,7 @@ tables:
 
 reporting:
   output_directory: "/home/user/reports"
-  investigation_directory: "/home/user/investigation_reports"
+
   formats: ["html", "csv"]
   filename_template: "test_regression_{timestamp}"