docs: add security report issue template

[EasterRobert]

Introduced a structured security report template to help users report vulnerabilities in a consistent and actionable format.

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Note

Low Risk
Adds a GitHub issue template only; no runtime code or security-sensitive logic changes. Risk is limited to minor workflow/triage impact.

Overview
Adds a new GitHub issue template security_report.md to standardize how users report vulnerabilities (detail, impact, repro steps, additional info) and auto-apply the security label with a [SECURITY] title prefix.

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Security reports via public issues enables vulnerability disclosure

Medium Severity

This template directs users to report security vulnerabilities through public GitHub issues. Security vulnerabilities should be reported privately to prevent attackers from exploiting them before fixes are available. The template even acknowledges the public nature by asking users not to share sensitive information publicly, but the fundamental issue remains that any reported vulnerability becomes immediately visible. Consider using GitHub's private vulnerability reporting feature or directing users to a SECURITY.md file with private contact information instead.