chore: consolidate docs into docs/, remove dead code

- docs/: moved BUILD_REPORT.md (from data/) and IMPROVEMENT_LOG.md + collection.md
  (from collection/); joins model_evaluation.md and silent_fix_detection.md.
- Removed validated-negative / dead-end scripts (findings preserved in docs):
  detect_silent_fixes.py (regex classifier), train_silent_fix_classifier.py
  (TF-IDF), enrich_linked_issues.py (linked-issue dead end).
- Updated all path references (README, docs, code).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: grandchildrice

README.md

@@ -96,7 +96,7 @@ pulls (NVD / OSV / RustSec / govulncheck), and an LLM STRIDE+CWE classifier. See
 
 ## Build report
 
-[`data/BUILD_REPORT.md`](data/BUILD_REPORT.md) records the before/after of every
+[`docs/BUILD_REPORT.md`](docs/BUILD_REPORT.md) records the before/after of every
 build stage for the current snapshot.
 
 ## License