feat: add com.atproto.repo.strongRef lexicon
Port strongRef lexicon from atproto with:
- Standalone lexicon with main def
- @doc on model for top-level description
- at-uri and cid formats
Tests passing (16/16)
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>