feat(skills): add plugin-style skill system for dynamic instruction l…

[likebeans]

Summary

Add a plugin-style skill system inspired by GitHub Copilot / Cursor Rules / AGENTS.md that allows agents to dynamically discover, load, and execute structured instructions and scripts.

Core Components

• SkillRegistry: Multi-directory scanning with hierarchical override
• SkillLoader: Progressive/lazy content loading with 32KB cap
• SkillMatcher: Keyword, path glob, and extensible semantic matching
• ResourceResolver + ScriptExecutor: Resource file access and safe script execution
• SkillSystem facade: Unified interface + 6 LangChain tools for agent integration

What's Included

• src/skills/ — Full skill system module
• src/skills/catalog/git_operations/ — Example skill with commit check script and templates
• src/streamlit_app.py — Sidebar "Skills" popover showing skill load status
• tests/skills/test_skill_system.py — 25 unit tests covering all components
• docs/Skill_System.md — Feature documentation

Testing

pytest tests/skills/test_skill_system.py -v
# 25 passed

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## Note
 
Thank you for building and maintaining this excellent toolkit — it's been a great learning resource and foundation for exploring LangGraph-based agent architectures. 
 
This is my first contribution to this project. I'd love to hear your feedback on the design and implementation. Happy to adjust anything based on your preferences and project direction.