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/commands/mark-decision.command.ts

@@ -0,0 +1,13 @@
+import { Command } from '@nestjs/cqrs';
+import { CommandResult } from './command-result';
+
+/** Marks a comment as a decision of its ticket (`on`) or unmarks it. */
+export class MarkDecisionCommand extends Command<CommandResult> {
+  constructor(
+    public readonly actorId: string,
+    public readonly commentId: string,
+    public readonly on: boolean,
+  ) {
+    super();
+  }
+}