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-command-and-control-beaconing/SKILL.md

@@ -1,12 +1,25 @@
 ---
 name: hunting-for-command-and-control-beaconing
-description: Detect C2 beaconing patterns in network traffic using frequency analysis, jitter detection, and domain reputation to identify compromised endpoints communicating with adversary infrastructure.
+description: Detect C2 beaconing patterns in network traffic using frequency analysis, jitter detection, and domain reputation
+  to identify compromised endpoints communicating with adversary infrastructure.
 domain: cybersecurity
 subdomain: threat-hunting
-tags: [threat-hunting, mitre-attack, c2, beaconing, network-analysis, proactive-detection]
-version: "1.0"
+tags:
+- threat-hunting
+- mitre-attack
+- c2
+- beaconing
+- network-analysis
+- proactive-detection
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+d3fend_techniques:
+- File Metadata Consistency Validation
+- Certificate Analysis
+- Application Protocol Command Analysis
+- Content Format Conversion
+- File Content Analysis
 ---
 
 # Hunting for Command and Control Beaconing