Document the `npm run drift-worker` CLI in the operator guide

[shivamlalakiya]

The operator guide's Drift queue (operator workflow) section (docs/operator-guide.md) explains the queue and the experimental LLM agent, but never mentions the npm run drift-worker script (scripts/drift-worker.ts) that operators/CI use to drain pending drift events.

Scope

• Document what npm run drift-worker does and when to run it
• Relate it to GET /api/drift/events?status=pending and the post-merge ack workflow
• Keep the human-review-required framing on the experimental LLM drift agent intact

Small, self-contained doc change — good first issue.