Agent: durable turn and queue state, visible inbox closure, OpenAI phase; prompt fixes; fetch/bench fixes

sdk/src/lib.rs

@@ -108,15 +108,17 @@ fn next_id() -> String {
 #[cfg(feature = "async")]
 pub struct Inbox {
     rx: async_channel::Receiver<Message>,
+    _reader: wstd::runtime::Task<()>,
 }
 
 #[cfg(feature = "async")]
 impl Inbox {
-    /// Start a task that reads stdin and returns an inbox for its messages.
+    /// Start the stdin reader inside the wstd runtime and return its inbox.
+    /// The inbox owns the reader task and must be dropped to cancel it.
     pub fn start() -> Self {
         let (tx, rx) = async_channel::unbounded();
-        wstd::runtime::spawn(read_messages(tx)).detach();
-        Self { rx }
+        let reader = wstd::runtime::spawn(read_messages(tx));
+        Self { rx, _reader: reader }
     }
 
     /// Wait for the next message, or return `None` at EOF or shutdown.
@@ -128,6 +130,11 @@ impl Inbox {
     pub fn try_recv(&self) -> Option<Message> {
         self.rx.try_recv().ok()
     }
+
+    /// Return whether the reader closed the inbox, even when messages remain buffered.
+    pub fn is_closed(&self) -> bool {
+        self.rx.is_closed()
+    }
 }
 
 #[cfg(feature = "async")]