chore: major repo cleanup - remove dead code and cruft

Remove ~24K lines of dead code for a lean rebuild foundation:

- Remove atomizer-field/ (neural field predictor experiment, concept archived in docs)
- Remove generated_extractors/, generated_hooks/ (legacy generator outputs)
- Remove optimization_validation/ (empty skeleton)
- Remove reports/ (superseded by optimization_engine/reporting/)
- Remove root-level stale files: DEVELOPMENT.md, INSTALL_INSTRUCTIONS.md,
  config.py, atomizer_paths.py, optimization_config.json, train_neural.bat,
  generate_training_data.py, run_training_fea.py, migrate_imports.py
- Update .gitignore for introspection caches and insight outputs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -127,5 +127,14 @@ backend_stderr.log
 # Auto-generated documentation (regenerate with: python -m optimization_engine.auto_doc all)
 docs/generated/
 
+# NX model introspection caches (generated)
+**/_introspection_*.json
+**/_introspection_cache.json
+**/_temp_introspection.json
+**/params.exp
+
+# Insight outputs (generated)
+**/3_insights/
+
 # Malformed filenames (Windows path used as filename)
 C:*