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/infrastructure/src/lib/mongo/read-model.schemas.ts

@@ -239,6 +239,10 @@ class CommentDocument {
 }
 export const CommentSchema = SchemaFactory.createForClass(CommentDocument);
 CommentSchema.index({ issueId: 1, createdAt: 1 });
+CommentSchema.index(
+  { projectId: 1, decidedAt: -1 },
+  { partialFilterExpression: { decision: true } },
+);
 
 @Schema({ ...LOOSE, collection: 'releases_view' })
 class ReleaseDocument {