Add SECURITY.md policy

[Galigar]

Added a standard security policy to guide researchers on how to privately report vulnerabilities instead of opening public issues.

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Note

Establishes a straightforward private vulnerability disclosure process.

• Adds SECURITY.md defining how to report vulnerabilities privately (email repo owners or create a draft security advisory)
• Specifies required report details: description, reproduction steps, and impact; discourages public GitHub issues; commits to timely response

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[github-actions]

Welcome to Polymarket Agents. Thank you for creating your first PR. Cheers!