Refactor _ai_github_utilities.py into modular package structure

[github-actions]

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I restructured the _ai_github_utilities.py module by creating a new directory structure with separate files for different functionalities. The main functions were split into comment_on_issue.py for issue commenting, solve_issue.py for issue solving logic, and utilities.py for shared GitHub API interaction functions. The original _ai_github_utilities.py file was updated to re-export all functions through the new init.py file, maintaining backwards compatibility while achieving better code organization and separation of concerns.

• src/git_bob/_ai_github_utilities/init.py Created new package initialization file that exports main functionality.
• src/git_bob/_ai_github_utilities/comment_on_issue.py Implemented dedicated module for GitHub issue commenting functionality.
• src/git_bob/_ai_github_utilities/solve_issue.py Created new module with issue solving logic and interfaces.
• src/git_bob/_ai_github_utilities/utilities.py Added shared utility functions for GitHub API interactions.
• src/git_bob/_ai_github_utilities.py Updated main module to import and re-export functions from new structure.

closes #451