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-end-to-end-encryption-for-messaging/SKILL.md

@@ -1,7 +1,8 @@
 ---
 name: implementing-end-to-end-encryption-for-messaging
-description: End-to-end encryption (E2EE) ensures that only the communicating parties can read messages, with no intermediary
-  (including the server) able to decrypt them. This skill implements a simplified version
+description: End-to-end encryption (E2EE) ensures that only the communicating parties
+  can read messages, with no intermediary (including the server) able to decrypt them.
+  This skill implements a simplified version
 domain: cybersecurity
 subdomain: cryptography
 tags:
@@ -17,6 +18,11 @@ nist_csf:
 - PR.DS-01
 - PR.DS-02
 - PR.DS-10
+mitre_attack:
+- T1600
+- T1573
+- T1553
+- T1486
 ---
 # Implementing End-to-End Encryption for Messaging