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

@@ -30,6 +30,9 @@ export class CommentAddedEventHandler
       replyTo: comment.replyTo ?? null,
       deleted: false,
       reactions: [],
+      decision: false,
+      decidedBy: null,
+      decidedAt: null,
       createdAt: comment.createdAt,
       editedAt: null,
       version: versionOf(event),