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/testing-for-email-header-injection/SKILL.md

@@ -1,12 +1,25 @@
 ---
 name: testing-for-email-header-injection
-description: Test web application email functionality for SMTP header injection vulnerabilities that allow attackers to inject additional email headers, modify recipients, and abuse contact forms for spam relay.
+description: Test web application email functionality for SMTP header injection vulnerabilities that allow attackers to inject
+  additional email headers, modify recipients, and abuse contact forms for spam relay.
 domain: cybersecurity
 subdomain: web-application-security
-tags: [email-injection, smtp-injection, crlf-injection, header-injection, spam-relay, contact-form, email-security]
-version: "1.0"
+tags:
+- email-injection
+- smtp-injection
+- crlf-injection
+- header-injection
+- spam-relay
+- contact-form
+- email-security
+version: '1.0'
 author: mahipal
 license: Apache-2.0
+nist_csf:
+- PR.PS-01
+- ID.RA-01
+- PR.DS-10
+- DE.CM-01
 ---
 
 # Testing for Email Header Injection