Fix generators not including extra_context (facets list)
The compose_instructions() function returns extra_context containing
facet summaries, but generators weren't using it. This caused the
facets generator to only see examples in the prompt rather than the
actual available facets, resulting in generic classifications like
"work" instead of specific facets like "sunstone" or "bluesky".
Now appends extra_context to system_instruction for generators.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>