Agent output must contain valid YAML frontmatter matching the role schema.
If frontmatter parsing fails, the step fails immediately with a clear error
instead of falling back to an LLM extraction that can fabricate values.
The extract module remains as a public API export but is no longer used
in the agent run loop.
Breaking change: agents that relied on LLM extraction to produce valid
output will now fail. They must output proper frontmatter.
xiaoju
99a2890be2