The credential proxy already reads ANTHROPIC_AUTH_TOKEN (credential-proxy.ts
line 33) and uses it for OAuth-mode authentication, but setup/verify.ts did
not include it in its credential-detection regex. Users with
ANTHROPIC_AUTH_TOKEN in .env saw 'CREDENTIALS: missing' even though their
credentials were valid at runtime.
Add ANTHROPIC_AUTH_TOKEN to the regex and add a matching test case.
Closes gh-853
