This PR introduces a standardized pull request template setup to improve
review quality and consistency.
## Summary
- Adds a new default PR template at `.github/pull_request_template.md`.
- Standardizes PR submissions with consistent sections for context and
validation.
- Establishes a single source of truth for PR guidance.
## Why
- A unified template improves review quality and consistency.
- It reduces ambiguity for contributors when describing changes and test
coverage.
- Centralizing guidance makes future maintenance simpler.
