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

@@ -1,7 +1,8 @@
 ---
 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:
@@ -21,6 +22,12 @@ nist_csf:
 - DE.CM-09
 - RS.CO-02
 - DE.AE-02
+mitre_attack:
+- T1566
+- T1598
+- T1534
+- T1036
+- T1027
 ---
 # Implementing Proofpoint Email Security Gateway