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/analyzing-powershell-empire-artifacts/SKILL.md

@@ -1,12 +1,32 @@
 ---
 name: analyzing-powershell-empire-artifacts
-description: Detect PowerShell Empire framework artifacts in Windows event logs by identifying Base64 encoded launcher patterns, default user agents, staging URL structures, stager IOCs, and known Empire module signatures in Script Block Logging events.
+description: Detect PowerShell Empire framework artifacts in Windows event logs by identifying Base64 encoded launcher patterns,
+  default user agents, staging URL structures, stager IOCs, and known Empire module signatures in Script Block Logging events.
 domain: cybersecurity
 subdomain: threat-hunting
-tags: [PowerShell-Empire, threat-hunting, Script-Block-Logging, base64, stager, C2, MITRE-ATT&CK, T1059.001, forensics]
-version: "1.0"
+tags:
+- PowerShell-Empire
+- threat-hunting
+- Script-Block-Logging
+- base64
+- stager
+- C2
+- MITRE-ATT&CK
+- T1059.001
+- forensics
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+d3fend_techniques:
+- Executable Denylisting
+- Execution Isolation
+- File Metadata Consistency Validation
+- Content Format Conversion
+- File Content Analysis
+nist_ai_rmf:
+- GOVERN-1.1
+- MEASURE-2.7
+- MANAGE-3.1
 ---
 
 # Analyzing PowerShell Empire Artifacts