docs: consolidate and clean up documentation

- Remove redundant files: AGENTS.md, INSTRUCTIONS.yaml, instructions.mdc
- Create comprehensive CONTRIBUTING.md with all development guidelines
- Add simple documentation index at docs/README.md
- Update main README to reference consolidated docs
- Remove implementation notes and release summaries
- Keep only essential, current documentation

.rules

@@ -0,0 +1,27 @@
+PROJECT: UnitForge
+STAGE: development
+PRODUCTION: false
+COMPATIBILITY: none
+PRINCIPLE: keep_small_simple
+
+RULE: fix_lint_errors
+RULE: keep_code_lean
+RULE: keep_tests_consistent
+RULE: remove_legacy_code
+RULE: use_makefile_services
+SERVER: uvicorn
+PACKAGE_MANAGER: uv
+
+CMD: make setup-dev
+CMD: make server
+CMD: make test
+CMD: make test-cov
+CMD: make lint && make format
+CMD: make dev
+
+STYLE: python_pep8
+STYLE: python_type_hints
+STYLE: python_formatter_black
+STYLE: code_no_duplication
+STYLE: code_high_cohesion
+STYLE: code_minimal_complexity