Update pull request template: require evidence, tests, and AI disclosure

Part of #6393.

Problem

The pull request template (.github/PULL_REQUEST_TEMPLATE.md) asks how a change was tested but does not prompt for visual evidence of working behavior, does not have a checkbox affirming tests were added/updated, and does not ask contributors to disclose AI assistance.

Proposed change

Update .github/PULL_REQUEST_TEMPLATE.md to add:

• An Evidence prompt in the Validation section requesting a screenshot or short recording demonstrating the change working (where applicable).
• A checklist item: Added or updated tests (or explained why not applicable).
• A checklist item: Disclosed any AI-assisted authoring (ties to the AI contribution policy sub-issue).

Keep the existing sections (Description, References, Validation, Checklist, Issue Type) intact and consistent with the current format.

Dependencies

• The AI disclosure checkbox should align with the wording from the AI-assisted contribution policy sub-issue.

Acceptance criteria

• PR template includes an evidence prompt, a tests checkbox, and an AI-assisted disclosure checkbox.

(Created with GitHub Copilot assistance.)

Open .github/PULL_REQUEST_TEMPLATE.md and compare its existing sections with the proposed changes. Add the Evidence prompt and the tests and AI-assisted disclosure checklist items, using wording aligned with the AI-assisted contribution policy sub-issue. Done means all three additions are present while the existing sections remain intact and consistent.