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-gcp-vpc-firewall-rules/SKILL.md

@@ -1,8 +1,9 @@
 ---
 name: implementing-gcp-vpc-firewall-rules
-description: 'Implementing and auditing GCP VPC firewall rules to enforce network segmentation, restrict ingress and egress
-  traffic, apply hierarchical firewall policies across the organization, and monitor firewall rule effectiveness using VPC
-  Flow Logs.
+description: 'Implementing and auditing GCP VPC firewall rules to enforce network
+  segmentation, restrict ingress and egress traffic, apply hierarchical firewall policies
+  across the organization, and monitor firewall rule effectiveness using VPC Flow
+  Logs.
 
   '
 domain: cybersecurity
@@ -22,6 +23,11 @@ nist_csf:
 - ID.AM-08
 - GV.SC-06
 - DE.CM-01
+mitre_attack:
+- T1078.004
+- T1530
+- T1537
+- T1580
 ---
 
 # Implementing GCP VPC Firewall Rules