Implement source-finder skill for Python package repository discovery

Description

Create a new Claude Code skill called "source-finder" in the python-packaging plugin that automatically discovers the source repository for any given Python package.

The skill should provide a deterministic approach to finding package repositories by:

1. Primary method: Check PyPI project metadata for repository links (homepage, repository, source, etc.)
2. Fallback method: Use web search when PyPI metadata is insufficient
3. Output: Return the repository URL with a confidence level (high/medium/low)

Acceptance Criteria

• Skill integrates with existing python-packaging plugin structure
• Checks PyPI API for project URLs and metadata
• Falls back to web search for packages without clear repository links
• Returns structured output with repository URL and confidence level
• Handles common edge cases (archived repos, redirects, multiple possible sources)
• Follows Claude Code skills documentation standards
• Passes make lint and make update validation

Technical Notes

• Focus on GitHub as primary target platform (most common)
• Consider GitLab, Bitbucket, and other platforms as secondary
• Implement confidence scoring based on source reliability (PyPI metadata > search results)