📝 docs: add ADR for staged-only Git hooks formatting

- Add ADR-0012 documenting the decision to format only staged Go files
- Update ADR README.md with new entry
- Document rationale, alternatives, and verification results
- Include future considerations for monitoring and CI/CD integration

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

pkg/greet/greet.go

@@ -21,4 +21,4 @@ func (s *Service) Greet(ctx context.Context, name string) string {
 		return "Hello world!"
 	}
 	return "Hello " + name + "!"
-}
+}