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/configuring-multi-factor-authentication-with-duo/SKILL.md

@@ -1,7 +1,8 @@
 ---
 name: configuring-multi-factor-authentication-with-duo
-description: Deploy Cisco Duo multi-factor authentication across enterprise applications, VPN, RDP, and SSH access points.
-  This skill covers Duo integration methods, adaptive authentication policies, device trust
+description: Deploy Cisco Duo multi-factor authentication across enterprise applications,
+  VPN, RDP, and SSH access points. This skill covers Duo integration methods, adaptive
+  authentication policies, device trust
 domain: cybersecurity
 subdomain: identity-access-management
 tags:
@@ -20,6 +21,12 @@ nist_csf:
 - PR.AA-02
 - PR.AA-05
 - PR.AA-06
+mitre_attack:
+- T1621
+- T1110.004
+- T1110.003
+- T1078
+- T1556.006
 ---
 # Configuring Multi-Factor Authentication with Duo