feat: what is new since my last visit, and turning a comment into a ticket

Opening a ticket notes the visit and answers the one before (issue_visits),
so the page marks others' comments and changes since. A comment can become
a ticket of its own, as a subtask or linked; it remembers where it went.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/application/src/lib/event-handlers/comment-added.event-handler.ts

@@ -43,6 +43,7 @@ export class CommentAddedEventHandler
       resolved: false,
       resolvedBy: null,
       resolvedAt: null,
+      spawned: [],
       createdAt: comment.createdAt,
       editedAt: null,
       version: versionOf(event),