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/performing-open-source-intelligence-gathering/SKILL.md

@@ -1,7 +1,8 @@
 ---
 name: performing-open-source-intelligence-gathering
-description: Open Source Intelligence (OSINT) gathering is the first active phase of a red team engagement, where operators
-  collect publicly available information about the target organization to identify attack s
+description: Open Source Intelligence (OSINT) gathering is the first active phase
+  of a red team engagement, where operators collect publicly available information
+  about the target organization to identify attack s
 domain: cybersecurity
 subdomain: red-teaming
 tags:
@@ -19,6 +20,12 @@ nist_csf:
 - ID.RA-01
 - GV.OV-02
 - DE.AE-07
+mitre_attack:
+- T1595
+- T1190
+- T1059
+- T1078
+- T1592
 ---
 # Performing Open Source Intelligence Gathering