feat: implement Phase 4 Security & Compliance (SOURCES.md, SECURITY.md) [skip ci]

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We track the `main` branch.
+
+## Reporting a Vulnerability
+
+**DO NOT** open a public Issue for security exploits.
+
+If you find a security vulnerability (e.g., a skill that bypasses the "Authorized Use Only" check or executes malicious code without warning):
+
+1. Email: `security@antigravity.dev` (Placeholder)
+2. Or open a **Private Advisory** on this repository.
+
+## Offensive Skills Policy
+
+Please read our [Security Guardrails](docs/SECURITY_GUARDRAILS.md).
+All offensive skills are strictly for **authorized educational and professional use only**.