Expand PR validation template

[mehul0810]

Summary

• Expands the PR template with linked issue, target branch, scope, non-goals, and user-facing impact prompts.
• Adds validation gates for PHPCS, PHP lint, PHPStan, PHPUnit, frontend lint/build, Playwright, WordPress smoke, and manual testing.
• Adds compatibility and contributor PR handling sections so replacement/salvage PRs preserve credit and explain why maintainer follow-up is needed.

Closes #158.

Validation

• git diff --check
• Compared against recent UI PR Add Perform dashboard overview #155: the new template would require screenshot/video proof or an explicit proof-not-applicable reason.
• Compared against tooling/docs PR Refresh AI agent repo guidance #160: the new template supports linked issue, narrow scope, no product-code changes, and skipped validation gates without excessive noise.
• Confirmed changed files are limited to .github/PULL_REQUEST_TEMPLATE.md; no product code changes are included.

Compatibility

• Settings/options/backward compatibility: Not applicable - PR template only.
• Public hooks or public PHP API: Not applicable - PR template only.
• Migrations and existing installs: Not applicable - PR template only.
• Multisite: Not applicable - PR template only.
• Supported PHP and WordPress versions: Not applicable - PR template only.
• Security/privacy-sensitive behavior: Not applicable - PR template only.
• Documentation/readme/changelog/release notes: GitHub PR template updated.

Screenshots or video

Not applicable - GitHub PR template text only.

Contributor PR handling

Not applicable - this PR does not replace a human contributor PR.

[github-actions]

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