refactor: implement self-registration pattern for language support

[soumyaray]

Refactor language filtering architecture to use self-registration pattern, eliminating hardcoded constants and improving maintainability.

Domain layer changes:

• CodeLanguage: Add self-registration system with @registry

  • Each language class declares extension (e.g., extension 'rb')
  • Auto-registers via ClassMethods.extension hook
  • Remove hardcoded LANGUAGE_EXTENSION hash
  • Add wanted_extensions public interface

• Streamline wanted?/unwanted? predicates

  • Single source of truth in ClassMethods
  • Instance methods delegate to class methods
  • Remove WANTED_LANGUAGES constant from FileContributions

• FilePath improvements

  • Add wanted? method for domain-level file filtering
  • Fix extension method with safe navigation for files without extensions
  • Add guard in language method to return Unknown for missing extensions

Infrastructure layer changes:

• LocalGitRepo: Remove utility function code smell
  • Remove hardcoded TEXT_FILES constant
  • Replace wanted_code_file? with Value::FilePath#wanted?
  • Infrastructure now queries domain for language support

Benefits:

• Single source of truth for language definitions
• Self-documenting: languages declare their own extensions
• Infrastructure automatically stays in sync with domain
• Eliminates utility function smell (reek clean)
• Domain logic properly encapsulated in domain layer

Tests: All passing (35 runs, 132 assertions, 0 failures)
Code quality: RuboCop and Reek clean

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