Add %%clusterfy IPython magic command with interactive widget interface

This major enhancement provides a powerful configuration management interface for Jupyter notebooks:

## New Features
- **%%clusterfy Magic Command**: Creates interactive widget for cluster configuration
- **Configuration Widget**: Full-featured GUI with dropdowns, buttons, and forms
- **Default Configurations**: Built-in templates for major cloud providers
- **Save/Load Functionality**: Import/export configurations as YAML or JSON
- **Auto-Registration**: Magic command loads automatically when importing clustrix

## Widget Capabilities
- Define and name cluster configurations with intuitive interface
- Switch between configurations with dropdown selector
- Apply configurations to current session with one click
- Comprehensive form fields for all cluster types (local, SSH, SLURM, Kubernetes)
- Smart field visibility based on cluster type selection
- Status messages and error handling

## Configuration Management
- Create, edit, and delete configurations
- Save configurations to files for sharing and version control
- Load configurations from existing files
- Built-in validation and error checking

## Default Templates Include
- Local development environment
- AWS GPU instances (small/large)
- GCP CPU instances
- Azure GPU instances
- SLURM HPC clusters
- Kubernetes clusters

## Technical Implementation
- Graceful fallback when IPython/ipywidgets not available
- Comprehensive test suite (19 tests, all passing)
- Clean separation of UI and logic components
- Type-safe configuration handling

## Usage
```python
import clustrix  # Auto-loads magic command
```

```
%%clusterfy
# Interactive widget appears
```

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

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

Author: jeremymanning

clustrix/__init__.py

@@ -35,3 +35,16 @@
     "CostEstimate",
     "CostReport",
 ]
+
+# Auto-register IPython magic command if in notebook environment
+try:
+    from IPython import get_ipython
+
+    ipython = get_ipython()
+    if ipython is not None:
+        from .notebook_magic import load_ipython_extension
+
+        load_ipython_extension(ipython)
+except (ImportError, AttributeError):
+    # Not in IPython/Jupyter environment
+    pass