Add a standardized pull request template (#4655)

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.

Author: fernandolins

.github/pull_request_template.md

@@ -0,0 +1,61 @@
+<!--
+Title (for the Pull Request title field at the top):
+Use a short prefix so the change type is obvious. You do not need to repeat it in the body below.
+
+Examples:
+- fix: — bugfix
+- feat: — feature
+- refactor: — internal change without user-facing feature
+- docs: — documentation only
+- chore: — tooling, CI, deps, build housekeeping
+- test: — tests only
+-->
+
+## Linked issue (required)
+
+<!-- Fixes #123 / Closes #123 / Refs #123 -->
+
+## Summary / motivation (required)
+
+<!-- What this PR does and why. For larger changes, add enough context for reviewers. -->
+
+## Steps to reproduce (required, use N/A if not applicable)
+
+<!-- Steps to reproduce: how to trigger the bug in the broken state (the "before").
+ - Mainly for bugfixes;
+    - For bugs: numbered steps before the fix. For non-bugs: write N/A.
+ - use N/A for features, refactors, docs, chore, etc.
+-->
+
+1.
+2.
+3.
+
+## How to test (required)
+
+<!--- How to test: how you verified the change (checks, unit tests, manual steps, edge cases — the "after" or general validation). --->
+
+### Checklist (minimum)
+
+- [ ] I ran `./ninja check` or an equivalent relevant check locally.
+- [ ] I added or updated tests when the change is non-trivial or behavior changed.
+
+### Details
+
+<!-- Commands, manual steps, edge cases, and what you observed -->
+
+## Before / after behavior (optional)
+
+<!-- For bugfixes: behavior before vs after. For other types: N/A or a short note. -->
+
+## Risk / compatibility / migration (optional)
+
+<!-- Breaking changes, rollout notes, or N/A for small / low-risk PRs -->
+
+## UI evidence (required for visual changes; otherwise N/A)
+
+<!-- Screenshot or short video -->
+
+## Scope
+
+- [ ] This PR is focused on one change (no unrelated edits).