fix(credentials): align priority tie-break with CPA

[Willxup]

Summary

• Keep numeric credential priority as the primary descending sort.
• Sort equal-priority Auth Files by the case-insensitive CPA file name so Keeper matches CPAMC ordering.
• Add regression coverage for same-email Codex credentials with account-hash file names.

Validation

• go test ./internal/repository/... -count=1
• go test ./cmd/... ./internal/...
• git diff --check

Fixes #356