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-security-information-sharing-with-stix2/SKILL.md

@@ -1,7 +1,8 @@
 ---
 name: implementing-security-information-sharing-with-stix2
-description: 'Create, validate, and share STIX 2.1 threat intelligence objects using the stix2 Python library. Covers indicators,
-  malware, campaigns, relationships, bundles, and TAXII 2.1 publishing.
+description: 'Create, validate, and share STIX 2.1 threat intelligence objects using
+  the stix2 Python library. Covers indicators, malware, campaigns, relationships,
+  bundles, and TAXII 2.1 publishing.
 
   '
 domain: cybersecurity
@@ -25,6 +26,12 @@ nist_csf:
 - ID.RA-05
 - DE.CM-01
 - DE.AE-02
+mitre_attack:
+- T1591
+- T1592
+- T1593
+- T1589
+- T1027
 ---
 
 # Implementing Security Information Sharing with STIX 2.1