Add shared skills support across all adapters

[donnes]

Summary

This PR introduces shared skills support that works consistently across all AI agent adapters.

Changes

• Added shared-skills.ts adapter with centralized shared skills handling
• Updated all existing adapters (Claude, Cursor, GitHub Copilot, Gemini CLI, OpenCode, Kimi CLI, Devin, Droid, AMP, Codex, VSCode) to support shared skills
• Added shared skills metadata and configuration types
• Enhanced new command with improved shared skills scaffolding
• Updated init, sync, and unsync commands to handle shared skills
• Added documentation updates for project structure

Files Changed

• Core adapter updates in src/adapters/
• Command enhancements in src/commands/
• Metadata and type updates in src/agents/ and src/config/

Features

• Consistent shared skills directory structure across all adapters
• Automatic symlink/copy workflows for shared skill files
• Adapter-specific shared skills path resolution
• Support for both agent-specific and shared skill configurations