Add CLAUDE.md for project guidance and enhance get_object_subtree_with_fields to handle circular references

- Introduced CLAUDE.md to provide comprehensive guidance on the Penpot MCP Server, including project overview, key commands, architecture, and common workflows.
- Enhanced the `get_object_subtree_with_fields` function to track visited nodes and handle circular references, ensuring robust tree structure retrieval.
- Added tests for circular reference handling in `test_penpot_tree.py` to validate new functionality.

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Penpot MCP Server is a Python-based Model Context Protocol (MCP) server that bridges AI language models with Penpot, an open-source design platform. It enables programmatic interaction with design files through a well-structured API.
+
+## Key Commands
+
+### Development Setup
+
+```bash
+# Install dependencies (recommended)
+uv sync --extra dev
+
+# Run the MCP server
+uv run penpot-mcp
+
+# Run tests
+uv run pytest
+uv run pytest --cov=penpot_mcp tests/  # with coverage
+
+# Lint and fix code
+uv run python lint.py              # check issues
+uv run python lint.py --autofix    # auto-fix issues
+```
+
+### Running the Server
+
+```bash
+# Default stdio mode (for Claude Desktop/Cursor)
+make mcp-server
+
+# SSE mode (for debugging with inspector)
+make mcp-server-sse
+
+# Launch MCP inspector (requires SSE mode)
+make mcp-inspector
+```
+
+### CLI Tools
+
+```bash
+# Generate tree visualization
+penpot-tree path/to/penpot_file.json
+
+# Validate Penpot file
+penpot-validate path/to/penpot_file.json
+```
+
+## Architecture Overview
+
+### Core Components
+
+1. **MCP Server** (`penpot_mcp/server/mcp_server.py`)
+   - Built on FastMCP framework
+   - Implements resources and tools for Penpot interaction
+   - Memory cache with 10-minute TTL
+   - Supports stdio (default) and SSE modes
+
+2. **API Client** (`penpot_mcp/api/penpot_api.py`)
+   - REST client for Penpot platform
+   - Transit+JSON format handling
+   - Cookie-based authentication with auto-refresh
+   - Lazy authentication pattern
+
+3. **Key Design Patterns**
+   - **Authentication**: Cookie-based with automatic re-authentication on 401/403
+   - **Caching**: In-memory file cache to reduce API calls
+   - **Resource/Tool Duality**: Resources can be exposed as tools via RESOURCES_AS_TOOLS config
+   - **Transit Format**: Special handling for UUIDs (`~u` prefix) and keywords (`~:` prefix)
+
+### Available Tools/Functions
+
+- `list_projects`: Get all Penpot projects
+- `get_project_files`: List files in a project
+- `get_file`: Retrieve and cache file data
+- `search_object`: Search design objects by name (regex)
+- `get_object_tree`: Get filtered object tree with screenshot
+- `export_object`: Export design objects as images
+- `penpot_tree_schema`: Get schema for object tree fields
+
+### Environment Configuration
+
+Create a `.env` file with:
+```
+PENPOT_API_URL=https://design.penpot.app/api
+PENPOT_USERNAME=your_username
+PENPOT_PASSWORD=your_password
+ENABLE_HTTP_SERVER=true  # for image serving
+RESOURCES_AS_TOOLS=false # MCP resource mode
+DEBUG=true               # debug logging
+```
+
+### Working with the Codebase
+
+1. **Adding New Tools**: Decorate functions with `@self.mcp.tool()` in mcp_server.py
+2. **API Extensions**: Add methods to PenpotAPI class following existing patterns
+3. **Error Handling**: Always check for `"error"` keys in API responses
+4. **Testing**: Use `test_mode=True` when creating server instances in tests
+5. **Transit Format**: Remember to handle Transit+JSON when working with raw API
+
+### Common Workflow for Code Generation
+
+1. List projects → Find target project
+2. Get project files → Locate design file  
+3. Search for component → Find specific element
+4. Get tree schema → Understand available fields
+5. Get object tree → Retrieve structure with screenshot
+6. Export if needed → Get rendered component image
+
+### Testing Patterns
+
+- Mock fixtures in `tests/conftest.py`
+- Test both stdio and SSE modes
+- Verify Transit format conversions
+- Check cache behavior and expiration