📝 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_v2_test.go

@@ -25,4 +25,4 @@ func TestServiceV2_GreetV2(t *testing.T) {
 			}
 		})
 	}
-}
+}