Allow setting an AI credit limit (--max-ai-credits) per automation

### Feature summary

A per-automation AI credit ceiling for automation runs, with clear handling when a run stops at the limit.

### What problem are you trying to solve?

Automations run unattended and can be triggered frequently. That is precisely the situation where an unbounded agent is most expensive: nobody is watching, so a run that goes off the rails keeps consuming credits until it finishes on its own.

There is currently no way to bound the cost of an automation run in the GitHub Copilot app. The CLI supports --max-ai-credits, but automations expose no setting for it.

A per-run cap on its own is also not sufficient for a scheduled automation. A cap of 100 credits per run still means 2,400 credits per day at hourly frequency. Bounding total spend for an automation matters as much as bounding a single run.

### Proposed solution

- Add a per-automation setting, e.g. "Maximum AI credits per run", mapping to --max-ai-credits=NUMBER.
- If a project-level default exists (see related request), automations should inherit it, with the ability to override per automation.
- When a run stops because it hit the limit, mark it distinctly in the run history — e.g. "Stopped: AI credit limit reached" — rather than reporting it as a plain failure or a normal completion. Otherwise a truncated result is indistinguishable from a finished one, which is worse than no limit at all.
- Notify on limit-stopped runs, since nobody is watching them live.
- Offer "Re-run with a higher limit" from the run detail view, so recovering from a too-low cap does not require editing the automation.
- Additionally, a budget across runs — e.g. a maximum number of AI credits per automation per calendar month, after which the automation pauses and notifies. For a frequently triggered automation this is the control that actually bounds spend.

### Workflow impact

Affects anyone running scheduled or frequently triggered automations, especially on usage-based billing.

Unlike an interactive session, a hard stop at the limit is acceptable here. There is no user present to be interrupted, and stopping is the desired outcome. What is needed is that the stop is visible and distinguishable, so a partial result is never mistaken for a complete one.

With a per-run cap and a per-automation budget, I can enable automations at a useful frequency with a known worst-case monthly cost, instead of running them conservatively or not at all.

### Installation context

_No response_

### Additional context

_No response_

Bắt đầu bằng cách xem xét các cài đặt tự động hóa hiện có và hành vi của CLI --max-ai-credits, sau đó theo dõi cách các lần chạy tự động hóa xuất hiện trong lịch sử, gửi thông báo và hỗ trợ chạy lại. Được xem là hoàn tất khi các giới hạn cho từng lần chạy và giữa các lần chạy, tính kế thừa và các ghi đè, trạng thái riêng biệt cho các lần chạy bị dừng do giới hạn, thông báo và việc chạy lại với giới hạn cao hơn được xác định trong các luồng này.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.