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/comment/comment.ts

@@ -5,6 +5,7 @@ import {
   assertThat,
 } from '@task-stream/api/shared/domain';
 import { CommentAddedEvent } from '../events/comment-added.event';
+import { CommentDecisionMarkedEvent } from '../events/comment-decision-marked.event';
 import { CommentDeletedEvent } from '../events/comment-deleted.event';
 import { CommentEditedEvent } from '../events/comment-edited.event';
 import { CommentReactedEvent } from '../events/comment-reacted.event';
@@ -41,6 +42,8 @@ export class Comment extends VersionedAggregateRoot implements CommentProps {
   public replyTo: string | null = null;
   /** Who chose each emoji (only the ones somebody chose). */
   public reactions: Record<string, string[]> = {};
+  /** What was decided on the ticket: shown above it and searchable. */
+  public decision = false;
   public deleted = false;
   public createdAt!: string;
   public editedAt: string | null = null;
@@ -93,6 +96,24 @@ export class Comment extends VersionedAggregateRoot implements CommentProps {
     );
   }
 
+  /**
+   * Marks it as a decision of the ticket (`on`) or unmarks it; repeating it
+   * changes nothing. Who may do it (the team) is the caller's check.
+   */
+  markDecision(actorId: string, on: boolean): void {
+    this.assertAlive();
+    if (on === this.decision) return;
+    this.apply(
+      new CommentDecisionMarkedEvent(
+        actorId,
+        this.id,
+        this.projectId,
+        this.issueId,
+        on,
+      ),
+    );
+  }
+
   /** @param canModerate whether the actor manages the project (may delete others' comments). */
   delete(actorId: string, canModerate: boolean): void {
     this.assertAlive();
@@ -121,6 +142,10 @@ export class Comment extends VersionedAggregateRoot implements CommentProps {
     this.reactions = event.reactions;
   }
 
+  onCommentDecisionMarkedEvent(event: CommentDecisionMarkedEvent): void {
+    this.decision = event.decision;
+  }
+
   onCommentDeletedEvent(): void {
     this.deleted = true;
   }