Enhancement: Add PR template with security and performance checklists

[AugistineCreates]

This pr closes #671

Summary

This PR introduces a standardized Pull Request template (.github/pull_request_template.md) and a basic CONTRIBUTING.md guide. The template includes essential checklists for security, performance, and general testing concerns to ensure a comprehensive review process prior to merging code into the repository.

It aims to prevent issues from slipping through the cracks, specifically regarding data storage, token handling, input validation, and rendering performance on React Native (useCallback, FlatList optimization). It also mandates checking whether an Architectural Decision Record (ADR) is present for architectural changes.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Chore / Refactor (no functional changes)

Checklist

• Added .github/pull_request_template.md with standard sections (Summary, Type of Change, Testing Done, Checklist).
• Added Security checklist (sensitive data storage, token exposure, input validation, deep linking).
• Added Performance checklist (memo/useCallback, FlatList optimizations, bundle size impacts).
• Added ADR checkbox for structural and architectural changes.
• Created CONTRIBUTING.md referencing the new template and onboarding guidelines.

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