Create SECURITY.md (#254)

docs: add SECURITY.md with responsible disclosure process

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported |
+|---------|-----------|
+| Latest  | ✅ Yes    |
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability in Agent-Reach, please report 
+it responsibly by using GitHub's private security advisory feature:
+
+👉 **[Report a vulnerability](https://github.com/Panniantong/Agent-Reach/security/advisories/new)**
+
+Please do NOT open a public GitHub issue for security vulnerabilities.
+
+## What to Include
+
+- Description of the vulnerability
+- Steps to reproduce
+- Affected versions
+- Potential impact
+- Suggested fix (if any)
+
+## Response Timeline
+
+- Acknowledgement within **48 hours**
+- Status update within **7 days**
+- Fix timeline communicated within **14 days**
+
+## Scope
+
+The following are considered in scope:
+- Authentication and authorization bypass
+- Remote code execution
+- Path traversal / arbitrary file read
+- Server-Side Request Forgery (SSRF)
+- Injection vulnerabilities (SQL, command, prompt)
+- Sensitive data exposure
+
+## Out of Scope
+
+- Vulnerabilities in dependencies (report to the dependency maintainer)
+- Social engineering attacks
+- Denial of service via resource exhaustion
+
+## Credits
+
+We appreciate responsible disclosure and will credit researchers 
+in our release notes unless anonymity is requested.