Add Mermaid workflow diagram generation and GitHub integration testing

[johnrwatson]

Summary

• Add new mermaid/workflow-diagram model for generating visual workflow diagrams
• Create comprehensive GitHub workflow for integration testing
• Test artifact passing, dependencies, and cross-model references

What's Added

• New Mermaid Model: Converts workflow execution data into Mermaid diagram format with color-coded status indicators
• Integration Test Workflow: End-to-end GitHub workflow that tests shell command dependencies, expression evaluation, and cross-model references
• Artifact Upload: Automatically uploads generated Mermaid diagrams and workflow execution artifacts

Test Coverage

• ✅ Shell command execution with dependencies
• ✅ Artifact passing between workflow steps
• ✅ Self-references using ${{ self.name }}
• ✅ Cross-model references using ${{ model.other-model.* }}
• ✅ Error handling and failure scenarios
• ✅ Mermaid diagram generation from workflow execution data

Test Plan

The new integration test workflow will:

1. Compile swamp binary and verify basic functionality
2. Create 5 models with interdependencies (download → process → summarize → final-report → diagram)
3. Execute a complex workflow testing all dependency patterns
4. Generate visual Mermaid diagrams of the workflow execution
5. Upload all artifacts for inspection
6. Validate error handling works correctly

🤖 Generated with Claude Code