feat: add NIST CSF 2.0 nist_csf field to all 754 cybersecurity skills

Mapped every skill to NIST CSF 2.0 subcategory IDs (GV/ID/PR/DE/RS/RC functions)
based on subdomain and content analysis. Restores 11 skills corrupted during
prior rebase, re-enriching with ATLAS, D3FEND, NIST AI RMF, and CSF 2.0 fields.

All 754 skills now carry structured mappings for all 5 security frameworks:
- MITRE ATT&CK (in tags)
- MITRE ATLAS v5.5 (atlas_techniques)
- MITRE D3FEND v1.3 (d3fend_techniques)
- NIST AI RMF 1.0 (nist_ai_rmf)
- NIST CSF 2.0 (nist_csf)

skills/implementing-beyondcorp-zero-trust-access-model/SKILL.md

@@ -1,15 +1,28 @@
 ---
 name: implementing-beyondcorp-zero-trust-access-model
-description: >
-  Implementing Google's BeyondCorp zero trust access model to eliminate implicit trust
-  from the network perimeter, enforce identity-aware access controls using IAP, Access
-  Context Manager, and Chrome Enterprise Premium for VPN-less secure application access.
+description: 'Implementing Google''s BeyondCorp zero trust access model to eliminate implicit trust from the network perimeter,
+  enforce identity-aware access controls using IAP, Access Context Manager, and Chrome Enterprise Premium for VPN-less secure
+  application access.
+
+  '
 domain: cybersecurity
 subdomain: zero-trust-architecture
-tags: [beyondcorp, zero-trust, google-cloud, iap, identity-aware-proxy, ztna, access-context-manager]
-version: "1.0"
+tags:
+- beyondcorp
+- zero-trust
+- google-cloud
+- iap
+- identity-aware-proxy
+- ztna
+- access-context-manager
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+nist_csf:
+- PR.AA-01
+- PR.AA-05
+- PR.IR-01
+- GV.PO-01
 ---
 
 # Implementing BeyondCorp Zero Trust Access Model