feat: forward AgentDelegation on PostMessage

Stage 8 Chat SDK support for Mira→Red Queen INVESTIGATE routing metadata.

Co-authored-by: Cursor <cursoragent@cursor.com>

client.go

@@ -134,6 +134,9 @@ type PostMessageRequest struct {
 	Content          string
 	ReplyToMessageID string
 	IdempotencyKey   string
+	// Optional machine-readable delegation metadata (AgentDelegation).
+	// Nil means no delegation; forwarded as-is to AgentPostMessage.
+	Delegation *pb.AgentDelegation
 }
 
 func (c *Client) PostMessage(ctx context.Context, req PostMessageRequest) (*pb.PostMessageResponse, error) {
@@ -144,6 +147,7 @@ func (c *Client) PostMessage(ctx context.Context, req PostMessageRequest) (*pb.P
 		TopicId:        req.TopicID,
 		Content:        req.Content,
 		IdempotencyKey: req.IdempotencyKey,
+		Delegation:     req.Delegation,
 	}
 	if req.ReplyToMessageID != "" {
 		replyTo := req.ReplyToMessageID

subscription_test.go

@@ -289,6 +289,42 @@ func TestPostMessageUsesAgentPostMessageAndPreservesIdempotencyKey(t *testing.T)
 	if len(fake.postReqs) != 1 || fake.postReqs[0].GetIdempotencyKey() != "logical-key" {
 		t.Fatalf("AgentPostMessage reqs = %+v", fake.postReqs)
 	}
+	if fake.postReqs[0].GetDelegation() != nil {
+		t.Fatalf("expected nil delegation, got %+v", fake.postReqs[0].GetDelegation())
+	}
+}
+
+func TestPostMessageForwardsDelegationMetadata(t *testing.T) {
+	fake := newFake()
+	c := newClientForTest(t, fake)
+	delegation := &pb.AgentDelegation{
+		TargetAuthorId: "22222222-2222-2222-2222-222222222222",
+		Kind:           "INVESTIGATE",
+	}
+	_, err := c.PostMessage(context.Background(), PostMessageRequest{
+		TopicID:        "topic",
+		Content:        "please investigate",
+		IdempotencyKey: "deleg-key",
+		Delegation:     delegation,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	fake.mu.Lock()
+	defer fake.mu.Unlock()
+	if fake.genericPosts != 0 {
+		t.Fatal("generic PostMessage must not be called")
+	}
+	if len(fake.postReqs) != 1 {
+		t.Fatalf("AgentPostMessage reqs = %+v", fake.postReqs)
+	}
+	got := fake.postReqs[0].GetDelegation()
+	if got == nil {
+		t.Fatal("expected delegation to be forwarded")
+	}
+	if got.GetTargetAuthorId() != delegation.TargetAuthorId || got.GetKind() != delegation.Kind {
+		t.Fatalf("delegation = %+v, want %+v", got, delegation)
+	}
 }
 
 func TestOrderedCumulativeACKWithSequenceGaps(t *testing.T) {