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-passwordless-authentication-with-fido2/SKILL.md

@@ -1,12 +1,29 @@
 ---
 name: implementing-passwordless-authentication-with-fido2
-description: Deploy FIDO2/WebAuthn passwordless authentication using security keys and platform authenticators. Covers WebAuthn API integration, FIDO2 server configuration, passkey enrollment, biometric authentica
+description: Deploy FIDO2/WebAuthn passwordless authentication using security keys and platform authenticators. Covers WebAuthn
+  API integration, FIDO2 server configuration, passkey enrollment, biometric authentica
 domain: cybersecurity
 subdomain: identity-access-management
-tags: [iam, identity, access-control, authentication, fido2, webauthn, passwordless]
-version: "1.0"
+tags:
+- iam
+- identity
+- access-control
+- authentication
+- fido2
+- webauthn
+- passwordless
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+atlas_techniques:
+- AML.T0051
+- AML.T0054
+- AML.T0056
+nist_ai_rmf:
+- MEASURE-2.7
+- MEASURE-2.5
+- GOVERN-6.1
+- MAP-5.1
 ---
 # Implementing Passwordless Authentication with FIDO2