Forward live usage stream events

The wire schema already allowed completed-turn usage, but rejected active usage updates before clients could see them. Accept usage_updated payloads and preserve turn ids on streamed turn events.

packages/protocol/src/messages.stream-parsing.test.ts

@@ -160,6 +160,45 @@ describe("shared messages stream parsing", () => {
     }
   });
 
+  it("parses agent_stream usage_updated events", () => {
+    const fixture = {
+      type: "agent_stream",
+      payload: {
+        agentId: "agent_live",
+        timestamp: "2026-02-08T20:10:00.000Z",
+        event: {
+          type: "usage_updated",
+          provider: "claude",
+          turnId: "foreground-turn-1",
+          usage: {
+            contextWindowUsedTokens: 24_074,
+            contextWindowMaxTokens: 200_000,
+          },
+        },
+      },
+    };
+
+    const parsed = AgentStreamMessageSchema.parse(fixture);
+    expect(parsed.payload.event).toEqual({
+      type: "usage_updated",
+      provider: "claude",
+      turnId: "foreground-turn-1",
+      usage: {
+        contextWindowUsedTokens: 24_074,
+        contextWindowMaxTokens: 200_000,
+      },
+    });
+
+    const wrapped = WSOutboundMessageSchema.parse({
+      type: "session",
+      message: fixture,
+    });
+    expect(wrapped.message.type).toBe("agent_stream");
+    if (wrapped.message.type === "agent_stream") {
+      expect(wrapped.message.payload.event.type).toBe("usage_updated");
+    }
+  });
+
   it("parses representative sub_agent tool_call event", () => {
     const parsed = AgentStreamMessageSchema.parse({
       type: "agent_stream",