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/mapping-mitre-attack-techniques/SKILL.md

@@ -1,17 +1,38 @@
 ---
 name: mapping-mitre-attack-techniques
-description: >
-  Maps observed adversary behaviors, security alerts, and detection rules to MITRE ATT&CK
-  techniques and sub-techniques to quantify detection coverage and guide control prioritization.
-  Use when building an ATT&CK-based coverage heatmap, tagging SIEM alerts with technique IDs,
-  aligning security controls to adversary playbooks, or reporting threat exposure to executives.
+description: 'Maps observed adversary behaviors, security alerts, and detection rules to MITRE ATT&CK techniques and sub-techniques
+  to quantify detection coverage and guide control prioritization. Use when building an ATT&CK-based coverage heatmap, tagging
+  SIEM alerts with technique IDs, aligning security controls to adversary playbooks, or reporting threat exposure to executives.
   Activates for requests involving ATT&CK Navigator, Sigma rules, MITRE D3FEND, or coverage gap analysis.
+
+  '
 domain: cybersecurity
 subdomain: threat-intelligence
-tags: [MITRE-ATT&CK, ATT&CK-Navigator, Sigma, D3FEND, TTP, detection-engineering, NIST-CSF]
+tags:
+- MITRE-ATT&CK
+- ATT&CK-Navigator
+- Sigma
+- D3FEND
+- TTP
+- detection-engineering
+- NIST-CSF
 version: 1.0.0
 author: mahipal
 license: Apache-2.0
+nist_ai_rmf:
+- MEASURE-2.7
+- MAP-5.1
+- MANAGE-2.4
+atlas_techniques:
+- AML.T0070
+- AML.T0066
+- AML.T0082
+d3fend_techniques:
+- Executable Denylisting
+- Execution Isolation
+- File Metadata Consistency Validation
+- Content Format Conversion
+- File Content Analysis
 ---
 # Mapping MITRE ATT&CK Techniques