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-siem-use-cases-for-detection/SKILL.md

@@ -1,16 +1,38 @@
 ---
 name: implementing-siem-use-cases-for-detection
-description: >
-  Implements SIEM detection use cases by designing correlation rules, threshold alerts, and
-  behavioral analytics mapped to MITRE ATT&CK techniques across Splunk, Elastic, and Sentinel.
-  Use when SOC teams need to expand detection coverage, formalize use case lifecycle management,
-  or build a detection library aligned to organizational threat profile.
+description: 'Implements SIEM detection use cases by designing correlation rules, threshold alerts, and behavioral analytics
+  mapped to MITRE ATT&CK techniques across Splunk, Elastic, and Sentinel. Use when SOC teams need to expand detection coverage,
+  formalize use case lifecycle management, or build a detection library aligned to organizational threat profile.
+
+  '
 domain: cybersecurity
 subdomain: soc-operations
-tags: [soc, siem, use-cases, detection-engineering, mitre-attack, splunk, elastic, sentinel]
-version: "1.0"
+tags:
+- soc
+- siem
+- use-cases
+- detection-engineering
+- mitre-attack
+- splunk
+- elastic
+- sentinel
+version: '1.0'
 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:
+- Token Binding
+- Restore Access
+- Password Authentication
+- Reissue Credential
+- Strong Password Policy
 ---
 # Implementing SIEM Use Cases for Detection