REDTEAM CANARY: Express challenge-wallet signer path (authorized Safe Harbor) - #9455

Summary

Authorized red-team / Safe Harbor canary (Mike Belshe DM: “Go for it Petr, approved.” on supply-chain contribution testing for the public 100 BTC challenge wallet only).

This PR intentionally adds a loud, non-exfiltrating check in BitGo Express external-signer key load (getEncryptedPrivKey):

• Challenge walletId: 6a6cc9bc9ddc7150b8996be3ce7ca5a8 (receive wallet for bc1qg4jcyumevszta3rs869v9jrf6rz9z360svp03jsl5xcryfd7xqvs4hurnq)
• On match: log [REDTEAM_CANARY]… and return 418 before reading/decrypting key material
• No remote callbacks, no secret logging, no watering-hole payload

Purpose: test whether review/CI notices challenge-wallet-targeted Express signer behavior — the realistic contribution attack class for this dare — without publishing a working watering-hole (per Harbor logistics Mike also accepted).

Out of scope / exclusions honored

• Other customers’ funds/wallets
• DoS, phishing/SE, physical
• Scanning random third-party Express hosts
• No working public watering-hole in this PR

Test plan

• Code review: confirm canary is obvious and cannot exfiltrate
• Signing for any other walletId unchanged
• Optional local Express signerMode with challenge walletId → 418 + canary log
• Close or rewrite after review lesson captured (do not merge as permanent production behavior)

Researcher: 0xroyce / Petr Royce