Add AI usage checkboxes to ai_agents_test issue template

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Problem/Motivation

Parent: #3568605: [Policy] Issue AI usage declaration policy

Issues must declare their AI usage, or they violate DA policies as noted in: Abuse of the Contribution Credit system

Proposed resolution

Update the issue template with checkboxes from the parent issue.

Target date or deadline

January 2026

Remaining tasks

• Update issue template
• Let people know about it