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

.claude/settings.json

@@ -0,0 +1,9 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(ffprobe -v error *)",
+      "Bash(env)",
+      "mcp__outline__read_document"
+    ]
+  }
+}