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-fileless-malware-techniques/SKILL.md

@@ -1,17 +1,28 @@
 ---
 name: detecting-fileless-malware-techniques
-description: >
-  Detects and analyzes fileless malware that operates entirely in memory using PowerShell,
-  WMI, .NET reflection, registry-resident payloads, and living-off-the-land binaries (LOLBins)
-  without writing traditional executable files to disk. Activates for requests involving
-  fileless threat detection, in-memory malware investigation, LOLBin abuse analysis, or
-  WMI persistence examination.
+description: 'Detects and analyzes fileless malware that operates entirely in memory using PowerShell, WMI, .NET reflection,
+  registry-resident payloads, and living-off-the-land binaries (LOLBins) without writing traditional executable files to disk.
+  Activates for requests involving fileless threat detection, in-memory malware investigation, LOLBin abuse analysis, or WMI
+  persistence examination.
+
+  '
 domain: cybersecurity
 subdomain: malware-analysis
-tags: [malware, fileless, LOLBins, memory-analysis, detection]
+tags:
+- malware
+- fileless
+- LOLBins
+- memory-analysis
+- detection
 version: 1.0.0
 author: mahipal
 license: Apache-2.0
+d3fend_techniques:
+- Executable Denylisting
+- Execution Isolation
+- File Metadata Consistency Validation
+- Content Format Conversion
+- File Content Analysis
 ---
 
 # Detecting Fileless Malware Techniques