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-proofpoint-email-security-gateway/SKILL.md

@@ -1,12 +1,26 @@
 ---
 name: implementing-proofpoint-email-security-gateway
-description: Deploy and configure Proofpoint Email Protection as a secure email gateway to detect and block phishing, malware, BEC, and spam before messages reach user inboxes.
+description: Deploy and configure Proofpoint Email Protection as a secure email gateway to detect and block phishing, malware,
+  BEC, and spam before messages reach user inboxes.
 domain: cybersecurity
 subdomain: phishing-defense
-tags: [email-security, proofpoint, secure-email-gateway, phishing, anti-spam, anti-malware, bec, email-filtering]
-version: "1.0"
+tags:
+- email-security
+- proofpoint
+- secure-email-gateway
+- phishing
+- anti-spam
+- anti-malware
+- bec
+- email-filtering
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+nist_csf:
+- PR.AT-01
+- DE.CM-09
+- RS.CO-02
+- DE.AE-02
 ---
 # Implementing Proofpoint Email Security Gateway