Map all 754 skills to MITRE ATT&CK v19.1

- Add validated mitre_attack frontmatter to all 754 skills (286 distinct
  techniques), verified against MITRE ATT&CK v19.1 via the official
  mitreattack-python library: 0 revoked, deprecated, or invalid IDs
- Curate precise per-skill technique IDs for forensics, malware-analysis,
  threat-intel, and red-team skills (e.g. DCSync -> T1003.006,
  Kerberoasting -> T1558.003, Pass-the-Ticket -> T1550.003)
- Reconcile v19.1 tactic restructuring: Defense Evasion split into
  Stealth (TA0005) and Defense Impairment (TA0112); revoked T1562.*
  family and T1070.001/.002 remapped to active equivalents (T1685.*)
- Normalize word-split tags across 35 skills (remove filename-derived
  stopword tags, add semantic cybersecurity tags)
- Add api-reference.md for 3 skills that were missing it
- Update README ATT&CK section with accurate v19.1 tactic distribution

skills/exploiting-ipv6-vulnerabilities/SKILL.md

@@ -1,7 +1,8 @@
 ---
 name: exploiting-ipv6-vulnerabilities
-description: 'Identifies and exploits IPv6-specific vulnerabilities including SLAAC spoofing, Router Advertisement flooding,
-  and IPv6 tunneling during authorized assessments to test dual-stack security controls and IPv6-aware network defenses.
+description: 'Identifies and exploits IPv6-specific vulnerabilities including SLAAC
+  spoofing, Router Advertisement flooding, and IPv6 tunneling during authorized assessments
+  to test dual-stack security controls and IPv6-aware network defenses.
 
   '
 domain: cybersecurity
@@ -20,6 +21,11 @@ nist_csf:
 - DE.CM-01
 - ID.AM-03
 - PR.DS-02
+mitre_attack:
+- T1046
+- T1040
+- T1557
+- T1071
 ---
 # Exploiting IPv6 Vulnerabilities