Add CLAUDE.md with build commands and code style guidelines

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

Author: devondragon

CLAUDE.md

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+# Spring User Framework - Developer Guide
+
+## Commands
+- **Build**: `./gradlew build`
+- **Run Tests**: `./gradlew test`
+- **Run Single Test**: `./gradlew test --tests "com.digitalsanctuary.spring.user.service.UserServiceTest"`
+- **Test with JDK17**: `./gradlew testJdk17`
+- **Test with JDK21**: `./gradlew testJdk21`
+- **Test All JDKs**: `./gradlew testAll`
+- **Lint/Check**: `./gradlew check`
+- **Publish Locally**: `./gradlew publishLocal`
+
+## Code Style Guidelines
+- **Imports**: Organize imports alphabetically, no wildcards
+- **Formatting**: Use proper indentation (4 spaces)
+- **Documentation**: Use JavaDoc for all public classes and methods
+- **Naming**: CamelCase for classes, lowerCamelCase for methods/variables
+- **Logging**: Use SLF4J (Lombok @Slf4j) with appropriate log levels
+- **Error Handling**: Use specific exceptions, provide meaningful messages
+- **Nullability**: Check for null before accessing potentially null references
+- **Services**: Use @RequiredArgsConstructor with final fields for DI