feat: Support automatic artifact storage for files generated by skill toolset scripts

## Summary
When skill toolset scripts generate files (e.g., PDF creation, image generation, data
exports), these files are created in temporary directories and lost after execution.
There's currently no built-in mechanism to persist skill-generated files as artifacts.

## Current Behavior
- Skill scripts can generate files in temporary directories during execution
- These files are not automatically saved as artifacts
- SaveFilesAsArtifactsPlugin only handles user-uploaded files (inline_data)
- Files are discarded after script execution completes

## Expected Behavior
One or more of:
1. Automatic artifact storage for skill-generated files (if returned in response)
2. Plugin support for intercepting tool responses with file data
3. Documentation on best practices for persisting skill-generated files
4. Skill API extension for explicit artifact saving

## Use Cases
- PDF generation skills that need to save generated PDFs
- Image processing skills that output images
- Data processing skills that generate reports or exports
- Any skill that creates files meant to be accessed later

## Example Scenario
```python
# PDF skill script generates a file
# Expected: File automatically becomes available as artifact
# Actual: File is lost after script execution