fix(workspace): add derives/docs per code review, remove unused thiserror from solitaire_sync

solitaire_gpgs/src/stub.rs

@@ -2,11 +2,14 @@ use async_trait::async_trait;
 use solitaire_data::{SyncError, SyncProvider};
 use solitaire_sync::{SyncPayload, SyncResponse};
 
-/// Desktop/iOS stub — always returns UnsupportedPlatform.
-/// Real implementation lives in android.rs (Phase: Android).
+/// Google Play Games Services sync client — desktop/iOS stub.
+///
+/// Always returns [`SyncError::UnsupportedPlatform`]. The real JNI implementation
+/// lives in `android.rs` and is compiled only on Android (Phase: Android).
 pub struct GpgsClient;
 
 impl GpgsClient {
+    /// Creates a new `GpgsClient` stub. No-op on non-Android platforms.
     pub fn new() -> Self {
         Self
     }