ios: add Agent attachments and shared transcript fade

ios/Packages/Sybil/Sources/Sybil/SybilAgentTransport.swift

@@ -13,6 +13,9 @@ protocol AgentAPIClienting: Sendable {
     func agentReadState() async throws -> AgentReadState
     func agentMarkRead(cursor: String) async throws -> AgentReadState
     func agentSubmit(content: String, requestID: String) async throws -> AgentReceipt
+    func agentSubmit(_ submission: AgentSubmission) async throws -> AgentReceipt
+    func agentUpload(file: AgentPreparedAttachment, uploadID: String) async throws -> AgentAttachment
+    func agentDownload(_ attachment: AgentAttachment) async throws -> Data
     func agentSocket(after: String) async throws -> any AgentSocket
 }
 
@@ -54,3 +57,34 @@ extension APIConfiguration {
         return String(decoding: try JSONEncoder().encode(value), as: UTF8.self)
     }
 }
+
+// Text-only test/legacy clients fail closed for the new wire path.
+extension AgentAPIClienting {
+    func agentSubmit(_ submission: AgentSubmission) async throws -> AgentReceipt {
+        guard submission.attachmentIDs.isEmpty else { throw AgentAttachmentIssue.unavailable }
+        return try await agentSubmit(content: submission.content, requestID: submission.requestId)
+    }
+    func agentUpload(file: AgentPreparedAttachment, uploadID: String) async throws -> AgentAttachment { throw AgentAttachmentIssue.unavailable }
+    func agentDownload(_ attachment: AgentAttachment) async throws -> Data { throw AgentAttachmentIssue.unavailable }
+}
+
+final class AgentAttachmentRedirectPolicy: NSObject, URLSessionTaskDelegate, Sendable {
+    // Reject every redirect; attachment credentials are valid only for the configured API.
+    func urlSession(_ session: URLSession, task: URLSessionTask, willPerformHTTPRedirection response: HTTPURLResponse,
+                    newRequest request: URLRequest, completionHandler: @escaping @Sendable (URLRequest?) -> Void) {
+        completionHandler(nil)
+    }
+}
+
+extension APIConfiguration {
+    func agentAttachmentContentURL(id: String) throws -> URL {
+        guard !id.isEmpty, var parts = URLComponents(url: baseURL, resolvingAgainstBaseURL: false),
+              ["http", "https"].contains(parts.scheme), parts.host != nil, parts.user == nil, parts.password == nil,
+              let escaped = id.addingPercentEncoding(withAllowedCharacters: CharacterSet(charactersIn: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_")) else { throw APIError.invalidBaseURL }
+        let base = parts.percentEncodedPath.trimmingCharacters(in: CharacterSet(charactersIn: "/"))
+        parts.percentEncodedPath = (base.isEmpty ? "" : "/" + base) + "/v1/agent/attachments/" + escaped + "/content"
+        parts.query = nil; parts.fragment = nil
+        guard let url = parts.url else { throw APIError.invalidBaseURL }
+        return url
+    }
+}