docs: add pull request template

.github/pull_request_template.md

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+<!--
+  Thanks for contributing! Fill out the sections below to help reviewers
+  understand the change. Delete sections that don't apply.
+-->
+
+## One Line Summary
+
+<!-- A single sentence describing what this PR does. -->
+
+## Motivation
+
+<!--
+  Why is this change being made? Link the Linear ticket and/or GitHub issue.
+  Example: SDK-1234, Fixes #123
+-->
+
+## Scope
+
+<!--
+  What is and isn't affected by this PR.
+-->
+
+- **Affected**:
+- **Not affected**:
+
+## Testing
+
+### Manual
+
+<!--
+  Step-by-step manual test plan. Include the editor (Classic / Block),
+  WordPress version, and any relevant plugin settings.
+-->
+
+1.
+2.
+
+### Unit / Integration
+
+<!--
+  Results from `composer test` (or the relevant suite). Note any new tests
+  added.
+-->
+
+- [ ] `composer test` passes locally
+
+## Affected code checklist
+
+- [ ] PHP plugin code (`v3/`)
+- [ ] `v2/` legacy code
+- [ ] JS / CSS assets
+- [ ] Build / CI
+- [ ] Tests
+- [ ] Documentation (README, `readme.txt`, etc.)
+
+## Checklist
+
+- [ ] Code follows existing style in the touched files
+- [ ] Tested manually in the relevant editor(s)
+  - [ ] Gutenberg/Block editor
+  - [ ] Classic editor
+- [ ] No new lint or test errors introduced
+- [ ] Linear ticket / GitHub issue linked above
+- [ ] `readme.txt` and plugin version bumped if user-facing (release PRs only)