feat: GitHub Code Search als Agent-Tool + Dashboard-Badges

- UseGitHubSearch Property im AgentRecord (Default: false)
- DB-Migration: use_github_search Spalte in agents
- Checkbox im Agent-Formular (Standard: unchecked)
- Tool nur aktiv wenn Agent-Checkbox UND GitHubToken konfiguriert
- includeGitHubSearch Parameter durch StreamChat/AddToolsToPayload
- Dashboard-Badges: 🔮 Auto-Kontext + 🐙 GitHub Search

Models/AgentRecord.cs

@@ -17,6 +17,7 @@ public class AgentRecord
     public bool UseUserInfo { get; set; } = true;
     public bool UseMemory { get; set; } = true;
     public bool UseAutoContext { get; set; } = true;
+    public bool UseGitHubSearch { get; set; }
     public bool UseWebSearch { get; set; }
     public string WebSearchVersion { get; set; } = "";
     public bool AllowUserChoice { get; set; } = true;