chore: add standard repo infrastructure (CHANGELOG, CONTRIBUTING, SECURITY, Makefile)

Author: omiinaya

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
+
+## [Unreleased]
+
+### Added
+- Standard repo infrastructure: CHANGELOG, CONTRIBUTING, SECURITY, Makefile, .github templates

CONTRIBUTING.md

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+# Contributing
+
+## Getting Started
+
+```bash
+git clone https://github.com/omiinaya/react-portfolio.git
+cd react-portfolio
+npm install
+```
+
+## Development
+
+- **Dev:** `npm run dev`
+- **Build:** `npm run build`
+- **Lint:** `npm run lint`
+
+## Commit Messages
+
+```
+type: concise subject
+```
+
+Types: `feat:`, `fix:`, `docs:`, `refactor:`, `chore:`, `test:`
+

Makefile

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+.PHONY: install dev build lint test clean
+
+install:
+	npm install
+
+dev:
+	npm run dev
+
+build:
+	npm run build
+
+lint:
+	npm run lint
+
+test:
+	npm test
+
+clean:
+	rm -rf node_modules .next dist
+

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Please do not open public issues for security vulnerabilities.
+
+Contact the maintainer via Discord or GitHub Issues with `security` label.