feat: AI architecture assistant — chat with your registry

[rajnavakoti]

Description

Add an AI-powered chat interface to the catalog UI that lets users ask natural language questions about their architecture registry. The assistant understands the full registry graph (elements, relationships, layers, domains) and can answer questions like "What systems does the Billing domain depend on?" or "Show me all deprecated components."

Acceptance Criteria

• Chat panel accessible from the sidebar or a floating button
• Supports natural language queries against the registry graph
• Returns structured answers with links to relevant registry elements
• Understands relationships: "what depends on X?", "what does X connect to?"
• Understands filters: "show me all draft APIs", "list external systems"
• Works with any vocabulary (reads registry-mapping.yaml for type/layer names)
• Conversation history persisted per session

Out of Scope

• Multi-user chat or collaboration features
• Modifying registry entries via chat (read-only for v1)
• Cloud-hosted LLM requirement — should support local models too