Automatisierungsvorschlaege nur mit n8n-Anbindung (#726)

An instance without n8n has no use for workflow proposals. New instance
setting automation.enabled ("this instance works with n8n"), off by default:
- off: automation_find/automation_propose refuse, no candidates are put to
  agents, and the escalation tool does not mention proposals at all;
- the switch sits in the settings card for external MCP servers; the
  approvals view says proposals are off instead of standing empty.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ClawdDotNet.Core/Config/InstanceConfig.cs

@@ -159,6 +159,10 @@ public sealed class InstanceConfig
     [JsonPropertyName("fileHistory")]
     public Files.FileHistorySettings FileHistory { get; set; } = new();
 
+    /// <summary>#726: Whether this instance works with n8n — only then do agents propose workflows. Off by default.</summary>
+    [JsonPropertyName("automation")]
+    public Automation.AutomationSettings Automation { get; set; } = new();
+
     /// <summary>#197: External MCP servers whose tools the agents may use — set up by a human only.</summary>
     [JsonPropertyName("mcpServers")]
     public List<Mcp.McpServerConfig> McpServers { get; set; } = new();