feat(agent): add opt-in push notifications

ios/Packages/Sybil/Sources/Sybil/SybilAgentModels.swift

@@ -73,6 +73,7 @@ struct AgentPage: Codable, Sendable {
 struct AgentReadState: Codable, Sendable {
     var cursor: String
     var updatedAt: String?
+    var sourceId: String? = nil
 }
 struct AgentReceipt: Codable, Sendable {
     var id: Int64
@@ -95,6 +96,7 @@ enum AgentEvent: Sendable {
     case ready(String?)
     case messages(AgentPage)
     case activity(String, AgentActivity)
+    case readState(AgentReadState)
     case heartbeat
     case error(String)
 
@@ -118,6 +120,13 @@ enum AgentEvent: Sendable {
             }
             return .activity(id, activity)
         case "pong", "heartbeat": return .heartbeat
+        case "read_state":
+            // Additive optional frame: malformed new metadata must not tear down
+            // an otherwise compatible Agent connection.
+            guard let read = try? AgentCoding.decoder().decode(AgentReadState.self, from: data),
+                  AgentPushPayload.validCursor(read.cursor) != nil,
+                  read.sourceId.map(AgentPushPayload.validSource) ?? true else { return nil }
+            return .readState(read)
         case "error": return .error(envelope.error ?? "Agent connection error.")
         default: return nil
         }