feat(studio): add YAML import entry

[xgtcode]

Summary

• Add a “Import YAML” entry to the Studio agent creation menu
• Support uploading .yaml / .yml files and restoring them into the custom creation flow with the existing yamlToDraft parser
• Add static regression coverage for the YAML import wiring

Changes

• Added an “Import YAML” card to the add-agent entry menu
• Added a hidden file picker with YAML extension validation
• On successful parse, route users into custom creation with customCreateMode set to yaml_import
• Show a parse error message when YAML import fails
• Updated codePackageDeploy.test.mjs to cover the import entry, accepted file types, and flow wiring

Test Plan

• .venv/bin/pre-commit run --all-files
• cd frontend && npm test
• cd frontend && npm run build