feat: enrich 209 skills with MITRE ATLAS, D3FEND, and NIST AI RMF frontmatter

Added structured security framework mappings to SKILL.md frontmatter across all applicable skills:
- atlas_techniques: MITRE ATLAS v5.5 AML.TXXXX IDs (81 skills, AI-targeted attack techniques)
- d3fend_techniques: MITRE D3FEND v1.3 defensive technique labels (139 skills, mapped from ATT&CK IDs)
- nist_ai_rmf: NIST AI RMF 1.0 subcategory IDs (85 skills, AI risk management functions)

Also updates ATTACK_COVERAGE.md with coverage statistics for all three frameworks.

skills/hunting-for-domain-fronting-c2-traffic/SKILL.md

@@ -1,19 +1,26 @@
 ---
 name: hunting-for-domain-fronting-c2-traffic
-description: Detect domain fronting C2 traffic by analyzing SNI vs HTTP Host header mismatches in proxy logs and TLS certificate discrepancies using pyOpenSSL for certificate inspection
+description: Detect domain fronting C2 traffic by analyzing SNI vs HTTP Host header mismatches in proxy logs and TLS certificate
+  discrepancies using pyOpenSSL for certificate inspection
 domain: cybersecurity
 subdomain: threat-hunting
 tags:
-  - domain-fronting
-  - c2-detection
-  - tls-inspection
-  - proxy-logs
-  - pyopenssl
-  - threat-hunting
-  - network-security
-version: "1.0"
+- domain-fronting
+- c2-detection
+- tls-inspection
+- proxy-logs
+- pyopenssl
+- threat-hunting
+- network-security
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+d3fend_techniques:
+- Application Protocol Command Analysis
+- Network Isolation
+- Network Traffic Analysis
+- Client-server Payload Profiling
+- Network Traffic Community Deviation
 ---
 
 # Hunting for Domain Fronting C2 Traffic