docs: mark FD-C07 Gist library as done

The gist library functionality is already implemented in formatrix-db:
- StoredDocument for gist storage
- Tags for categorization
- Full-text search
- Recent files list
- Graph edges for document links

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Jonathan D.A. Jewell

docs/MOSCOW-REQUIREMENTS.adoc

@@ -161,7 +161,7 @@ This document defines Must/Should/Could requirements for each component of the d
 
 |FD-C07
 |Gist library (ArangoDB-backed)
-|🔲 Pending
+|✅ Done
 
 |FD-C08
 |Share/send functionality