chore: add PR template, guidance, and gitar rule for quality checks

[Bueller87]

Which sample(s) or area?

.github, .gitar (repo tooling)

What changed?

Added a minimal PR template, a guidance doc that explains how to fill it out, and a gitar rule that evaluates PR descriptions against that guidance.
Why?

Automate PR quality so contributors get a clear, lightweight template and reviewers get consistent context (what changed, why, how it was tested). The guidance keeps the template short while documenting expectations (e.g. Gradle commands, when to use N/A). The gitar rule enforces those expectations on PR description creation/update.

How did you test it?

• Confirmed files exist and content matches the plan: .github/pull_request_template.md, .github/pull_request_guidance.md, .gitar/rules/pr-description-quality.md
• No workflow or code changes; additive only.

Potential risks

N/A — tooling only; no runtime or build impact.

Release notes

N/A — internal process improvement.

Documentation Changes

N/A — these files are the new documentation for PR process; no updates to README or sample docs required.

[gitar-bot]

Code Review ✅ Approved

Adds PR template, guidance, and gitar rule to establish quality check standards for the repository. No issues found.

Rules ✅ All requirements met

Repository Rules

✅ PR Description Quality Standards: All required sections contain substantive content and N/A is correctly applied for non-applicable sections.

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