docs(mcp-server): add SKILL.md for Claude Code integration

Add a skill file that enables Claude Code to effectively use the
Forest Admin MCP tools. The skill provides:

- Overview of all available tools and their purposes
- Workflow guidance for querying, modifying data, and executing actions
- Dynamic forms and multi-page action handling
- Filter syntax with common operators
- Practical examples for common operations
- getActionForm/executeAction response formats
- File upload/download handling
- Tips and known limitations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: alban bertolini