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/presenters/src/lib/http/dto/responses.dto.ts

@@ -625,12 +625,30 @@ export class CommentDto {
     description: 'In the order they are offered; only the ones somebody chose',
   })
   reactions!: CommentReactionDto[];
+  @ApiProperty({
+    description: 'Marked as a decision of the ticket (shown above it)',
+  })
+  decision!: boolean;
+  @ApiProperty({
+    type: UserRefDto,
+    nullable: true,
+    description: 'Who marked it as a decision',
+  })
+  decidedBy!: UserRefDto | null;
+  @ApiProperty({ type: String, nullable: true, format: 'date-time' })
+  decidedAt!: string | null;
   @ApiProperty({ format: 'date-time' }) createdAt!: string;
   @ApiProperty({ type: String, nullable: true, format: 'date-time' })
   editedAt!: string | null;
   @ApiProperty() version!: number;
 }
 
+/** A comment marked as a decision, with its ticket. */
+export class DecisionDto {
+  @ApiProperty({ type: CommentDto }) comment!: CommentDto;
+  @ApiProperty({ type: IssueParentDto }) issue!: IssueParentDto;
+}
+
 export class CommentPageDto {
   @ApiProperty({ type: [CommentDto] }) items!: CommentDto[];
   @ApiProperty() total!: number;