[codex] make scope.hpp API comments Doxygen-ready

[FIERsity]

Summary

Improve the API documentation in scope.hpp so the public declarations can be consumed more effectively by Doxygen.

The added documentation covers:

• scope_exit, scope_fail, and scope_success;
• unique_resource and make_unique_resource_checked;
• scope-guard concepts and construction behavior;
• scope_guard constructors, destruction, release, and deduction guides;
• the invocation-checker types used by the public aliases.

This change only updates comments and does not alter runtime behavior.

Validation

• compiled the header with GCC 16 in C++20 mode;
• syntax-checked all non-module examples:
  • scope_example.cpp
  • unique_resource.cpp
  • unique_resource_file.cpp
• git diff --check

Doxygen, CMake, and pre-commit were not available in the local environment, so generated documentation and the full project workflow were not run locally.

Fixes #38

[JeffGarland]

Sorry for the delayed review @FIERsity -- thank you for your contribution!

[JeffGarland]

one suggestion -- otherwise LGTM

[coveralls]

coverage: 100.0%. remained the same — FIERsity:codex/doxygen-scope-api-docs into bemanproject:main