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-zero-trust-with-beyondcorp/SKILL.md

@@ -1,8 +1,9 @@
 ---
 name: implementing-zero-trust-with-beyondcorp
-description: Deploy Google BeyondCorp Enterprise zero trust access controls using Identity-Aware Proxy (IAP), context-aware
-  access policies, device trust validation, and Access Context Manager to enforce identity and posture-based access to GCP
-  resources and internal applications.
+description: Deploy Google BeyondCorp Enterprise zero trust access controls using
+  Identity-Aware Proxy (IAP), context-aware access policies, device trust validation,
+  and Access Context Manager to enforce identity and posture-based access to GCP resources
+  and internal applications.
 domain: cybersecurity
 subdomain: zero-trust
 tags:
@@ -20,6 +21,12 @@ nist_csf:
 - PR.AA-01
 - PR.AA-05
 - PR.IR-01
+mitre_attack:
+- T1078
+- T1550
+- T1021
+- T1556
+- T1078.004
 ---
 
 # Implementing Zero Trust with BeyondCorp