fix: remove 6 duplicate stub files + add 37 technique spec mitigation back-references

Duplicates removed (stub files superseded by fuller versions):
- 2021-12-big-daddy-ape-club.md → 2022-01-big-daddy-ape-club.md
- 2022-04-elephant-money.md → 2022-04-elephant-money-flash-loan-governance-attack.md
- 2022-05-fortress-protocol.md → 2022-05-fortress-protocol-flash-loan-governance-attack.md
- 2025-counterfeit-ledger-nano-s-plus-cohort.md → 2025-01-counterfeit-ledger-nano-s-plus-cohort.md
- 2025-Q1-hayden-davis-kelsier-cluster.md → 2025-01-hayden-davis-kelsier-cluster.md
- 2018-big-pump-signal-telegram-cohort.md → 2017-01-big-pump-signal-telegram-cohort.md

Mitigation symmetry: 142 gaps → 0 across 37 technique spec YAML files.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: iZonex