[copa-sync] Add support for the copa generate workflow

[github-actions]

Summary

The v0.12.0 Copacetic release introduced the copa generate command for Docker Build integration (see https://github.com/project-copacetic/copacetic/releases/tag/v0.12.0), but this action still hardcodes copa patch. We need a way to trigger copa generate so users can pipe the generated build context into docker buildx build --platform ... --push ... directly from the workflow.

Details

• Determine the UX for choosing between patch and generate (new input, auto-detection, or dedicated step)
• When running generate, expose outputs (tarball path, VEX file) so downstream steps can run docker build with any required flags
• Document examples in README and add tests that cover both commands
• Ensure compatibility with existing inputs (reports, pkg-types, VEX output) and multi-platform builds

Acceptance Criteria

• New input (or workflow) allows switching to copa generate
• Action surfaces the generated context to subsequent workflow steps without requiring users to reimplement Docker piping manually
• README and tests demonstrate the new flow

Severity: Normal

AI generated by Copa Feature Sync for #55