- build_context: when conversation_type is 'briefing', inject a system
prompt instruction telling the model to answer from conversation history
and article context instead of searching the web
- Consolidate briefing conversation type detection to one DB query (was
being checked twice — once for the system prompt addition, once for
article context injection)
- ChatPanel: render a visual 'New Briefing Update' separator line before
2nd+ briefing slot messages (identified by metadata.rss_item_ids)
- types/chat.ts: add metadata field to Message interface
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>