fix(cli): handle empty YAML prompts

[RGBuilds]

Summary

• preserve empty and comment-only YAML prompt files as strings after parsing
• prevent evaluation from crashing when prompt rendering expects Prompt.raw to be a string
• correct the existing empty-document regression test

Testing

• npx vitest run test/prompts (16 files, 169 tests passed)
• npm run local -- eval -c <comment-only-yaml fixture> --no-cache -o <output.json> (1 passed, 0 failed, 0 errors)
• pre-commit Biome lint checks pass