fix(task): resolve thread root in createAgentComment for reply triggers

With trigger_comment_id now correctly pointing to the actual reply
(not the thread root), createAgentComment must resolve to the thread
root before posting. Otherwise error/system comments would have
parent_id pointing to a nested reply, making them invisible in the
frontend's flat thread grouping.

Part of MUL-708

server/internal/handler/comment.go

@@ -409,12 +409,9 @@ func (h *Handler) enqueueMentionedAgentTasks(ctx context.Context, issue db.Issue
 		if err != nil || hasPending {
 			continue
 		}
-		// Resolve thread root for reply threading.
-		replyTo := comment.ID
-		if comment.ParentID.Valid {
-			replyTo = comment.ParentID
-		}
-		if _, err := h.TaskService.EnqueueTaskForMention(ctx, issue, agentUUID, replyTo); err != nil {
+		// Always use the current comment as the trigger so the agent reads the
+		// actual reply that mentioned it, not the thread root.
+		if _, err := h.TaskService.EnqueueTaskForMention(ctx, issue, agentUUID, comment.ID); err != nil {
 			slog.Warn("enqueue mention agent task failed", "issue_id", uuidToString(issue.ID), "agent_id", m.ID, "error", err)
 		}
 	}

server/internal/service/task.go

@@ -561,6 +561,13 @@ func (s *TaskService) createAgentComment(ctx context.Context, issueID, agentID p
 	if err != nil {
 		return
 	}
+	// Resolve thread root: if parentID points to a reply (has its own parent),
+	// use that parent instead so the comment lands in the top-level thread.
+	if parentID.Valid {
+		if parent, err := s.Queries.GetComment(ctx, parentID); err == nil && parent.ParentID.Valid {
+			parentID = parent.ParentID
+		}
+	}
 	// Expand bare issue identifiers (e.g. MUL-117) into mention links.
 	content = mention.ExpandIssueIdentifiers(ctx, s.Queries, issue.WorkspaceID, content)
 	comment, err := s.Queries.CreateComment(ctx, db.CreateCommentParams{