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-email-sandboxing-with-proofpoint/SKILL.md

@@ -1,12 +1,25 @@
 ---
 name: implementing-email-sandboxing-with-proofpoint
-description: Email sandboxing detonates suspicious attachments and URLs in isolated environments to detect zero-day malware and evasive phishing payloads. Proofpoint Targeted Attack Protection (TAP) is an industry
+description: Email sandboxing detonates suspicious attachments and URLs in isolated environments to detect zero-day malware
+  and evasive phishing payloads. Proofpoint Targeted Attack Protection (TAP) is an industry
 domain: cybersecurity
 subdomain: phishing-defense
-tags: [phishing, email-security, social-engineering, dmarc, awareness, sandboxing, proofpoint]
-version: "1.0"
+tags:
+- phishing
+- email-security
+- social-engineering
+- dmarc
+- awareness
+- sandboxing
+- proofpoint
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+nist_csf:
+- PR.AT-01
+- DE.CM-09
+- RS.CO-02
+- DE.AE-02
 ---
 # Implementing Email Sandboxing with Proofpoint