chore: replace local skills module with git submodule

[PPeitsch]

⚡ Have you read the Contributing Guidelines?

Fixes #

Description

Replaced the local .agents/skills/ directory with the remote PPeitsch/skills submodule to improve code reuse and maintainability. Also updated AGENT.md to reflect this new usage and to enforce the execution of CI checks prior to commits.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would break existing functionality)
• Documentation update
• Performance improvement
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• CI/CD related changes
• Test coverage improvement
• Other (please describe):

Checklist

• I have followed the project's coding style guidelines
• I have added tests to cover my changes
• All new and existing tests pass locally
• I have updated the documentation accordingly
• I have added type hints where applicable
• I have updated the CHANGELOG.md
• My changes generate no new mypy warnings
• I have checked code formatting with black (line length 88)
• I have run isort for import sorting
• My PR is based on the latest main branch and has no conflicts
• I have added/updated docstrings in NumPy format

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.