Add Cloudflare's security-audit skill

Vendored from cloudflare/security-audit-skill under .agents/skills,
pinned in skills-lock.json and linked into .claude/skills. Also adds
the project's shared permission allow-rules in .claude/settings.json.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Tk3nAVF6n4dtjQS17FRFr

skills-lock.json

@@ -0,0 +1,11 @@
+{
+  "version": 1,
+  "skills": {
+    "security-audit": {
+      "source": "cloudflare/security-audit-skill",
+      "sourceType": "github",
+      "skillPath": "skills/security-audit/SKILL.md",
+      "computedHash": "98b97aad2873b3b9a8e064007c25e7827b493ba27fda8bfbdb41dab586767973"
+    }
+  }
+}