docs: add pull request template

[li041]

Summary

Add .github/pull_request_template.md to standardize pull request descriptions.

The template covers:

• Change summary
• Motivation and background
• Verification results
• Risks and limitations

Motivation

Existing pull requests may omit important context such as test results, affected platforms, or known limitations. This template
provides a consistent structure to improve review efficiency and completeness.

This is a documentation-only change and does not affect runtime behavior, ABI, or supported platforms.

Verification

• Confirmed .github/pull_request_template.md works and contains all required sections.
• No build or runtime tests were run because this change only adds documentation.

Risks and Limitations

• No runtime or compatibility risks are expected.
• Contributors may adjust or remove sections that are not applicable.

[agicy]

LGTM.