docs(contrib): add PR template and hygiene baseline (#724)

.github/pull_request_template.md

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+## Summary
+-
+
+## Why
+-
+
+## Issue
+- Closes #
+
+## Scope
+-
+
+## Out of scope
+-
+
+## Testing
+### Ran
+- `...`
+
+### Result
+- pass/fail:
+- if fail, exact files/errors:
+
+## CI status
+- pass:
+- code-related failures:
+- external/env/auth blockers:
+
+## Manual verification
+1.
+2.
+3.
+
+## Evidence
+- video/screenshot link, or `N/A (docs-only)`
+
+## Risk
+-
+
+## Rollback
+-