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-network-flow-data-with-netflow/SKILL.md

@@ -1,8 +1,10 @@
 ---
 name: analyzing-network-flow-data-with-netflow
-description: Parse NetFlow v9 and IPFIX records to detect volumetric anomalies, port scanning, data exfiltration, and C2 beaconing
-  patterns. Uses the Python netflow library to decode flow records, builds traffic baselines, and applies statistical analysis
-  to identify flows with abnormal byte counts, connection durations, and periodic timing patterns.
+description: Parse NetFlow v9 and IPFIX records to detect volumetric anomalies, port
+  scanning, data exfiltration, and C2 beaconing patterns. Uses the Python netflow
+  library to decode flow records, builds traffic baselines, and applies statistical
+  analysis to identify flows with abnormal byte counts, connection durations, and
+  periodic timing patterns.
 domain: cybersecurity
 subdomain: network-security
 tags:
@@ -18,6 +20,11 @@ nist_csf:
 - DE.CM-01
 - ID.AM-03
 - PR.DS-02
+mitre_attack:
+- T1071
+- T1048
+- T1046
+- T1095
 ---