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/analyzing-powershell-empire-artifacts/SKILL.md

@@ -1,7 +1,8 @@
 ---
 name: analyzing-powershell-empire-artifacts
-description: Detect PowerShell Empire framework artifacts in Windows event logs by identifying Base64 encoded launcher patterns,
-  default user agents, staging URL structures, stager IOCs, and known Empire module signatures in Script Block Logging events.
+description: Detect PowerShell Empire framework artifacts in Windows event logs by
+  identifying Base64 encoded launcher patterns, default user agents, staging URL structures,
+  stager IOCs, and known Empire module signatures in Script Block Logging events.
 domain: cybersecurity
 subdomain: threat-hunting
 tags:
@@ -32,6 +33,12 @@ nist_csf:
 - DE.AE-02
 - DE.AE-07
 - ID.RA-05
+mitre_attack:
+- T1059.001
+- T1071.001
+- T1003.001
+- T1558.003
+- T1027.010
 ---
 
 # Analyzing PowerShell Empire Artifacts