Add PR template

[askmy-stack]

Background

Identified during OSS audit v2 of Py-Outlier.

Problem Statement

Missing PR template.

Proposed Solution

.github/PULL_REQUEST_TEMPLATE.md.

Acceptance Criteria

• Implementation complete
• Tests pass
• Documented in README or docs/

Files / Modules Affected

.github/

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Suggested labels: priority:low, good first issue, category:documentation
Milestone: v1.0-oss-ready
OSS Audit: Master Backlog v2