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/implementing-security-information-sharing-with-stix2/SKILL.md

@@ -1,15 +1,25 @@
 ---
 name: implementing-security-information-sharing-with-stix2
-description: >
-  Create, validate, and share STIX 2.1 threat intelligence objects using
-  the stix2 Python library. Covers indicators, malware, campaigns,
-  relationships, bundles, and TAXII 2.1 publishing.
+description: 'Create, validate, and share STIX 2.1 threat intelligence objects using the stix2 Python library. Covers indicators,
+  malware, campaigns, relationships, bundles, and TAXII 2.1 publishing.
+
+  '
 domain: cybersecurity
 subdomain: threat-intelligence
-tags: [stix, taxii, threat-sharing, intelligence-exchange]
-version: "1.0"
+tags:
+- stix
+- taxii
+- threat-sharing
+- intelligence-exchange
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+d3fend_techniques:
+- File Metadata Consistency Validation
+- Application Protocol Command Analysis
+- Identifier Analysis
+- Content Format Conversion
+- Message Analysis
 ---
 
 # Implementing Security Information Sharing with STIX 2.1