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/web/tracker/application/src/lib/facades/issue.facade.ts

@@ -231,6 +231,11 @@ export class IssueFacade {
     this.dispatch.reactRequested({ commentId, emoji, on });
   }
 
+  /** Marks a comment as a decision of the ticket (`on`) or unmarks it. */
+  markDecision(commentId: string, on: boolean): void {
+    this.dispatch.decisionRequested({ commentId, on });
+  }
+
   /** A copy of what the open ticket says; answers its key. */
   duplicate(): Observable<string | null> {
     const issueId = this.store.issue()?.id ?? '';