docs(security): enable GitHub Security Advisories instructions (Fixes #3276) (#3311)

- Updated SECURITY.md with explicit instructions for reporting vulnerabilities via GitHub Security Advisories
- Replaced 'when available' qualifier since Private Vulnerability Reporting is now enabled
- Provided step-by-step guidance for security researchers to submit vulnerability reports

Co-authored-by: clawdbot <clawdbot@ultraworkers.dev>

SECURITY.md

@@ -8,10 +8,16 @@ unless a maintainer explicitly marks them as supported.
 
 ## Reporting a vulnerability
 
-Please do **not** open a public issue for a suspected vulnerability. Use GitHub
+Please do **not** open a public issue for a suspected vulnerability. Use [GitHub
-private vulnerability reporting for `ultraworkers/claw-code` when available, or
+Security Advisories](https://docs.github.com/en/code-security/security-advisories)
-contact a maintainer through the repository's published support channel with a
+for `ultraworkers/claw-code`:
-minimal, non-destructive reproduction.
+
+1. Go to the **Security** tab of this repository
+2. Click **"Report a vulnerability"**
+3. Describe the issue with reproduction steps and impact
+
+Alternatively, contact a maintainer through the repository's published support
+channel with a minimal, non-destructive reproduction.
 
 Include: