Add AI usage/automation policy

[bact]

Recent issues and PRs are likely to be generated mechanically.
We may need to have clear policy on this.

My personal view on this:

• In general, we should not against a well-understood technology that can offload some of our repetitive work.
• At the same time, we should be guarded against massive amount of geneated issues and PRs that will drain maintainers' capacity to properly review.
• At the very least, I would suggest that the issue/PR summary should be written by human. So it will express genuine concern for an issue or demonstrate understanding of the proposed changes. Any use of AI tool should be declared (the use of Copilot review is explicit in itself, as GitHub interface will show it as reviewer or in some case a co-author).

[wannaphong]

Good point. 👍

Now, We still don't get any spam PR from AI but we should make the policy.