docs: add SECURITY.md

Author: BuyWhere

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported |
+|---------|-----------|
+| 0.3.x   | ✅ |
+| < 0.3   | ❌ |
+
+## Reporting a Vulnerability
+
+Report security vulnerabilities to **security@buywhere.ai**. We respond within 48 hours.
+
+## API Key Security
+
+- Never commit `BUYWHERE_API_KEY` to version control
+- Use environment variables as shown in the README
+- Rotate keys regularly via the dashboard at https://buywhere.ai/api-keys