feat: comments marked as decisions, on top of the ticket and searchable

Whoever can change the ticket flags a comment as a decision (new event
CommentDecisionMarkedEvent on the comment). The ticket page lists its
decisions above the description, with a link to the comment in the
conversation (`?comment=<id>` scrolls to it and points at it), and the
search palette finds them by their text across my projects.

API: markDecision / PUT-DELETE /api/comments/{id}/decision; Issue.decisions
and GET /api/issues/{issue}/decisions; decisions(projectId?, search) and
GET /api/projects/{project}/decisions, GET /api/me/decisions. MCP:
mark_decision, search_decisions, and get_issue includes them.

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

libs/api/tracker/domain/src/lib/read-models/comment.read-model.ts

@@ -21,7 +21,18 @@ export class CommentReadModel {
   deleted!: boolean;
   /** In the order they are offered; only the ones somebody chose. */
   reactions!: CommentReactionReadModel[];
+  /** Marked as a decision of the ticket. */
+  decision!: boolean;
+  /** Who marked it and when (null when it is not a decision). */
+  decidedBy!: UserRefReadModel | null;
+  decidedAt!: string | null;
   createdAt!: string;
   editedAt!: string | null;
   version!: number;
 }
+
+/** A decision with the ticket it belongs to, for searching across tickets. */
+export class DecisionReadModel {
+  comment!: CommentReadModel;
+  issue!: { id: string; key: string; title: string };
+}