clients/swift: fix Linux compile — URLSession async API isn't on swift-corelibs-foundation

The audit-fix landing at 7e878e6f assumed URLSession.data(for:) and
URLSession.upload(for:fromFile:) are available on Linux Swift 5.9+.
They aren't — those are Apple-platform-only (macOS 12+, iOS 15+).
Linux's swift-corelibs-foundation only exposes the callback-style API.

Fix: private URLSession.forgeData(for:) / forgeUpload(for:fromFile:)
helpers on URLSession that bridge the callback API via
withCheckedThrowingContinuation on Linux (#if canImport(FoundationNetworking))
and forward to the native async API on Apple. 3 call sites updated.

Also added @preconcurrency to Foundation imports — URL, URLSession,
JSONEncoder, JSONDecoder are not Sendable on Linux Foundation;
@preconcurrency suppresses the spurious warnings without changing
runtime behavior.

Verified locally on Linux Swift 5.9.2 inside the crafting-table image
(7.8GB monolith with the full toolchain). The audit-fix agent that
shipped 7e878e6f was running without a swift toolchain and explicitly
flagged this verification was needed; this commit closes that.

clients/swift/Sources/Clawdforge/ForgeClient.swift

@@ -12,16 +12,68 @@
 // `URLSession.data(for:)` / `URLSession.upload(for:fromFile:)`, so wrapping
 // a call in `Task { ... }.cancel()` cleanly aborts the in-flight request.
 //
-// Linux: builds against swift-corelibs-foundation. The async URLSession
-// methods used here (`data(for:)`, `upload(for:fromFile:)`) are available
-// on Linux as of Swift 5.9.
+// Linux: builds against swift-corelibs-foundation. As of Swift 5.9.2 the
+// async `URLSession.data(for:)` / `URLSession.upload(for:fromFile:)` are
+// **only** on Apple platforms (macOS 12+, iOS 15+, etc.) — Linux's
+// FoundationNetworking exposes the callback-style URLSession API only.
+// The `forgeData(for:)` / `forgeUpload(for:fromFile:)` helpers below
+// bridge the callback API with `withCheckedThrowingContinuation` on Linux
+// and forward to the native async API on Apple.
+//
+// `@preconcurrency` on the Foundation imports suppresses Sendable warnings
+// for URL / URLSession / JSONEncoder / JSONDecoder which are not declared
+// Sendable on swift-corelibs-foundation; the SDK still preserves Sendable
+// semantics by the way it uses them (one shared session, immutable
+// captured fields).
 
-import Foundation
+@preconcurrency import Foundation
 
 #if canImport(FoundationNetworking)
-import FoundationNetworking
+@preconcurrency import FoundationNetworking
 #endif
 
+// MARK: - URLSession Linux/Apple bridge -----------------------------------
+
+extension URLSession {
+    /// Async `(Data, URLResponse)` for a request — uses the native API on
+    /// Apple, bridges the callback API on Linux. Behaves identically.
+    fileprivate func forgeData(for request: URLRequest) async throws -> (Data, URLResponse) {
+        #if canImport(FoundationNetworking)
+        return try await withCheckedThrowingContinuation { cont in
+            let task = self.dataTask(with: request) { data, response, error in
+                if let error { cont.resume(throwing: error); return }
+                guard let data, let response else {
+                    cont.resume(throwing: URLError(.unknown)); return
+                }
+                cont.resume(returning: (data, response))
+            }
+            task.resume()
+        }
+        #else
+        return try await self.data(for: request)
+        #endif
+    }
+
+    /// Async upload from a file — uses the native API on Apple, bridges
+    /// the callback API on Linux.
+    fileprivate func forgeUpload(for request: URLRequest, fromFile fileURL: URL) async throws -> (Data, URLResponse) {
+        #if canImport(FoundationNetworking)
+        return try await withCheckedThrowingContinuation { cont in
+            let task = self.uploadTask(with: request, fromFile: fileURL) { data, response, error in
+                if let error { cont.resume(throwing: error); return }
+                guard let data, let response else {
+                    cont.resume(throwing: URLError(.unknown)); return
+                }
+                cont.resume(returning: (data, response))
+            }
+            task.resume()
+        }
+        #else
+        return try await self.upload(for: request, fromFile: fileURL)
+        #endif
+    }
+}
+
 /// Thread-safe client for the clawdforge REST API.
 ///
 /// Construct once per (baseURL, token) pair and share across the app.
@@ -164,7 +216,7 @@ public struct ForgeClient: Sendable {
 
         let (data, response): (Data, URLResponse)
         do {
-            (data, response) = try await session.upload(for: req, fromFile: tempURL)
+            (data, response) = try await session.forgeUpload(for: req, fromFile: tempURL)
         } catch let urlError as URLError {
             throw ForgeError.transport(urlError)
         } catch {
@@ -275,7 +327,7 @@ public struct ForgeClient: Sendable {
 
         let (data, response): (Data, URLResponse)
         do {
-            (data, response) = try await session.data(for: req)
+            (data, response) = try await session.forgeData(for: req)
         } catch let urlError as URLError {
             throw ForgeError.transport(urlError)
         } catch {
@@ -291,7 +343,7 @@ public struct ForgeClient: Sendable {
 
         let (data, response): (Data, URLResponse)
         do {
-            (data, response) = try await session.data(for: req)
+            (data, response) = try await session.forgeData(for: req)
         } catch let urlError as URLError {
             throw ForgeError.transport(urlError)
         } catch {