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

[yesudeep]

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