Add runn failure diagnostics

[udzura]

Summary

• write Cloudflare runn exec responses to diagnostic files instead of dumping all steps
• upload the files only when the Cloudflare CI job fails
• ignore local diagnostic output

Verification

• YAML parsing
• git diff --check
• minimal runn failure scenario preserves stdout, stderr, and exit code in the response artifact

The full local Cloudflare runbook is blocked before project generation by existing macOS host-linking failures in workspace spike crates.