Refactors main function and enhances test suite

Refactors the main function for improved testability by extracting
the core logic into a new run function. Updates argument handling
and error reporting to use return codes instead of os.Exit.

Adds comprehensive test coverage for main functionality,
including integration tests and validation against edge cases.

Enhances README with updated code coverage and feature improvement lists.

Addresses improved maintainability and testability of the application.

Bumps version to 0.3.1

.gitignore

@@ -48,9 +48,12 @@ build/
 # Test coverage files
 coverage.out
 coverage.txt
+coverage.html
+coverage.*
 coverage
 *.cover
 *.coverprofile
+main_coverage
 
 # Other common exclusions
 *.exe