fix(presence): dedupe instances via stable instanceId

apps/macos/Sources/Clawdis/InstanceIdentity.swift

@@ -1,13 +1,24 @@
 import Foundation
 
 enum InstanceIdentity {
+    private static let suiteName = "com.steipete.clawdis.shared"
+    private static let instanceIdKey = "instanceId"
+
+    private static let defaults: UserDefaults = {
+        UserDefaults(suiteName: suiteName) ?? .standard
+    }()
+
     static let instanceId: String = {
-        if let name = Host.current().localizedName?.trimmingCharacters(in: .whitespacesAndNewlines),
-           !name.isEmpty
+        if let existing = defaults.string(forKey: instanceIdKey)?
+            .trimmingCharacters(in: .whitespacesAndNewlines),
+            !existing.isEmpty
         {
-            return name
+            return existing
         }
-        return UUID().uuidString
+
+        let id = UUID().uuidString.lowercased()
+        defaults.set(id, forKey: instanceIdKey)
+        return id
     }()
 
     static let displayName: String = {
@@ -19,4 +30,3 @@ enum InstanceIdentity {
         return "clawdis-mac"
     }()
 }
-