Add MITRE Fight Fraud Framework (F3 v1.1) mappings to fraud-relevant skills

- Add mitre_f3 frontmatter block to 94 fraud-relevant skills (phishing,
  account takeover, banking malware, BEC, identity/KYC, payment/card fraud,
  money-mule/cash-out, ransomware extortion, DFIR, threat intel)
- Map each skill to F3 v1.1 tactics + precise technique IDs, including the
  two F3-specific tactics ATT&CK lacks: Positioning (FA0001) and
  Monetization (FA0002)
- All 123 F3 v1.1 technique IDs validated against the upstream STIX bundle
  (github.com/center-for-threat-informed-defense/fight-fraud-framework):
  0 invalid IDs, 0 invalid tactics, 0 name mismatches, no placeholder IDs
- mitre_f3 kept as a separate block from mitre_attack (F3 redefines several
  ATT&CK tactics for the fraud context)
- Add docs/mitre-f3-mapping.md schema reference
- Update README: F3 as the 6th framework, dedicated F3 section + badge

skills/implementing-proofpoint-email-security-gateway/SKILL.md

@@ -28,6 +28,38 @@ mitre_attack:
 - T1534
 - T1036
 - T1027
+mitre_f3:
+  version: '1.1'
+  tactics:
+  - reconnaissance
+  - initial-access
+  - stealth
+  - positioning
+  techniques:
+  - id: T1598
+    name: Phishing for Information
+    tactic: reconnaissance
+    source: attack
+  - id: T1660
+    name: Phishing
+    tactic: initial-access
+    source: attack
+  - id: T1672
+    name: Email Spoofing
+    tactic: stealth
+    source: attack
+  - id: F1032
+    name: Impersonate Official
+    tactic: initial-access
+    source: f3
+  - id: F1029
+    name: Gather Customer Information
+    tactic: reconnaissance
+    source: f3
+  - id: F1005.006
+    name: 'Account Manipulation: Change of Payment Details'
+    tactic: positioning
+    source: f3
 ---
 # Implementing Proofpoint Email Security Gateway