Phase 4: create metadata files

[teresa-ortega]

Proposed changes

Add YAML metadata file generation to each benchmark iteration output folder.
At context creation, _write_metadata() writes metadata.yaml with run identity, variant parameters, options, source path, output paths, and start timestamp.

Type of change

• 🐛 Bugfix (change which fixes an issue)
• 🚀 Feature (change which adds functionality)
• 📚 Documentation (change which fixes or extends documentation)

💥 No breaking changes

Checklist

Put an x in the boxes that apply. This is simply a reminder of what we will require before merging your code.

• Lint and unit tests (if any) pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Additional comments

N/A