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/performing-active-directory-vulnerability-assessment/SKILL.md

@@ -1,12 +1,27 @@
 ---
 name: performing-active-directory-vulnerability-assessment
-description: Assess Active Directory security posture using PingCastle, BloodHound, and Purple Knight to identify misconfigurations, privilege escalation paths, and attack vectors.
+description: Assess Active Directory security posture using PingCastle, BloodHound, and Purple Knight to identify misconfigurations,
+  privilege escalation paths, and attack vectors.
 domain: cybersecurity
 subdomain: vulnerability-management
-tags: [active-directory, pingcastle, bloodhound, purple-knight, ad-security, privilege-escalation, ldap, kerberos]
-version: "1.0"
+tags:
+- active-directory
+- pingcastle
+- bloodhound
+- purple-knight
+- ad-security
+- privilege-escalation
+- ldap
+- kerberos
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+d3fend_techniques:
+- Restore Object
+- Network Traffic Policy Mapping
+- Restore Configuration
+- Access Modeling
+- Operational Activity Mapping
 ---
 
 # Performing Active Directory Vulnerability Assessment