feat(lexicon): add sources array to external embed for megathreads master
Adds optional sources field to social.coves.embed.external lexicon to support aggregator megathreads that combine multiple news sources. Changes: - Add #source definition with uri, title, domain, and optional sourcePost - Add sources array (max 50) to #external for aggregated links - Add maxLength constraints to domain (253) and provider (100) fields - Update descriptions to clarify primary vs aggregated content This enables LLM aggregators to create megathread posts that reference multiple source articles, with optional strongRef to existing Coves posts for future feed deprioritization. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>