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-agentless-vulnerability-scanning/SKILL.md

@@ -1,12 +1,25 @@
 ---
 name: performing-agentless-vulnerability-scanning
-description: Configure and execute agentless vulnerability scanning using network protocols, cloud snapshot analysis, and API-based discovery to assess systems without installing endpoint agents.
+description: Configure and execute agentless vulnerability scanning using network protocols, cloud snapshot analysis, and
+  API-based discovery to assess systems without installing endpoint agents.
 domain: cybersecurity
 subdomain: vulnerability-management
-tags: [agentless-scanning, vulnerability-assessment, cloud-security, ssh, wmi, snapshot-analysis, vuls, tenable]
-version: "1.0"
+tags:
+- agentless-scanning
+- vulnerability-assessment
+- cloud-security
+- ssh
+- wmi
+- snapshot-analysis
+- vuls
+- tenable
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+nist_ai_rmf:
+- GOVERN-1.1
+- MEASURE-2.7
+- MANAGE-3.1
 ---
 # Performing Agentless Vulnerability Scanning