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/detecting-email-forwarding-rules-attack/SKILL.md

@@ -1,12 +1,26 @@
 ---
 name: detecting-email-forwarding-rules-attack
-description: Detect malicious email forwarding rules created by adversaries to maintain persistent access to email communications for intelligence collection and BEC attacks.
+description: Detect malicious email forwarding rules created by adversaries to maintain persistent access to email communications
+  for intelligence collection and BEC attacks.
 domain: cybersecurity
 subdomain: threat-hunting
-tags: [threat-hunting, mitre-attack, email-forwarding, persistence, bec, t1114, proactive-detection]
-version: "1.0"
+tags:
+- threat-hunting
+- mitre-attack
+- email-forwarding
+- persistence
+- bec
+- t1114
+- proactive-detection
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+d3fend_techniques:
+- Restore Object
+- Restore Configuration
+- Application Configuration Hardening
+- Application Hardening
+- Disable Remote Access
 ---
 
 # Detecting Email Forwarding Rules Attack