fix: add is_sheltered attribute to AgentSnapshot class

- Add is_sheltered parameter to AgentSnapshot in both locations
- Include is_sheltered in agent_data dictionary for idle chat

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

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

unity-client/Assets/Scripts/Models.cs

@@ -59,6 +59,9 @@ namespace TheIsland.Models
         // Relationship 2.0 (Phase 17-B)
         public string social_role;  // "leader", "follower", "loner", "neutral"
 
+        // Shelter System (Phase 20-B)
+        public bool is_sheltered;
+
         public bool IsAlive => status == "Alive";
     }