v0.2.0 async and multi-folder discovery

• ✅ Full async/await support (adiscover(), ainvoke_skill())
• ✅ Multi-source skill discovery (project, Anthropic config, plugins, custom paths)
• ✅ Plugin ecosystem with MCPB manifest support
• ✅ Priority-based conflict resolution
• ✅ Fully qualified skill names (plugin:skill-name)
• ✅ Nested directory structures (up to 5 levels)
• ✅ Secure file path resolution with traversal prevention
• ✅ LangChain async integration
• ✅ Updated examples (async_usage.py, multi_source.py, file_references.py)
• ✅ Backward compatible with v0.1