Decide on error handling for None resolver returns and list-only frontmatter

## Summary

There are open questions about error handling in the Python implementation:

1. Should the resolver raise LookupError when it returns None?
2. Should list-only frontmatter (no key-value pairs) be an error?

## Locations

- `python/dotpromptz/src/dotpromptz/resolvers.py:146`: `# TODO: Should we raise a LookupError if the resolver returns None?`
- `python/dotpromptz/src/dotpromptz/parse.py:277`: `# TODO: Should this be an error?`

## Acceptance Criteria

- [ ] Document expected behavior in spec
- [ ] Implement consistent error handling across runtimes
- [ ] Add tests for edge cases