Add folder anatomy (scripts/agent.py + references/api-reference.md) for 648 cybersecurity skills

Complete skill folder anatomy across all cybersecurity skills:
- scripts/agent.py: 80-150 line Python agents using real libraries (impacket,
  boto3, azure-mgmt-*, kubernetes, pefile, yara, scapy, shodan, stix2, etc.)
- references/api-reference.md: real API documentation with method signatures
- LICENSE: MIT license for all skill folders

skills/performing-dns-tunneling-detection/references/api-reference.md

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+# API Reference: Performing DNS Tunneling Detection
+
+## scapy (DNS Packet Analysis)
+
+```python
+from scapy.all import rdpcap, DNS, DNSQR, DNSRR, sniff
+
+# Read from PCAP
+packets = rdpcap("traffic.pcap")
+for pkt in packets:
+    if pkt.haslayer(DNSQR):
+        qname = pkt[DNSQR].qname.decode()
+        qtype = pkt[DNSQR].qtype  # 1=A, 16=TXT, 28=AAAA
+
+# Live capture
+def dns_callback(pkt):
+    if pkt.haslayer(DNSQR):
+        print(pkt[DNSQR].qname)
+
+sniff(filter="udp port 53", prn=dns_callback, count=100)
+```
+
+## Shannon Entropy Calculation
+
+```python
+import math
+from collections import Counter
+
+def shannon_entropy(data):
+    counter = Counter(data)
+    length = len(data)
+    return -sum((c/length) * math.log2(c/length)
+                for c in counter.values())
+
+# Normal domain: ~2.5-3.5 bits
+# DNS tunnel:    ~4.0-5.0 bits
+```
+
+## DNS Tunneling Indicators
+
+| Indicator | Threshold | Rationale |
+|-----------|-----------|-----------|
+| Subdomain entropy | > 3.8 | Encoded/encrypted data |
+| Query length | > 50 chars | Payload in subdomain |
+| TXT queries/domain | > 20/hour | Data channel |
+| Unique subdomains | > 50/parent | Encoded sessions |
+| Digit ratio | > 0.4 | Base64/hex encoding |
+
+## Common DNS Tunnel Tools
+
+| Tool | Encoding | Record Types |
+|------|----------|-------------|
+| iodine | Base128 | NULL, TXT, CNAME |
+| dnscat2 | Hex/CNAME | TXT, MX, CNAME |
+| dns2tcp | Base64 | TXT, KEY |
+
+### References
+
+- scapy: https://scapy.readthedocs.io/en/latest/
+- DNS tunneling detection: https://www.sans.org/white-papers/dns-tunneling/
+- iodine: https://github.com/yarrick/iodine