refactor: turn MyString into a production-grade C++17 library

[Jyatin]

Summary

• Reorganize the project into include/src/examples/tests
• Implement the full Rule of Five with safe move semantics
• Add capacity management, append, search, substr, reverse, comparisons and checked access
• Add CMake build/test configuration
• Add GitHub Actions CI with ASan/UBSan
• Add comprehensive tests and a substantially improved README
• Remove obsolete root-level implementation/demo files

Validation

CI is configured to build the library and demo, run CTest, and run the tests under AddressSanitizer and UndefinedBehaviorSanitizer.