Test coverage gap

bin/utils/error.ts exports PakeError and isPakeError with no dedicated tests. isPakeError drives the clean user-facing error path in bin/cli.ts (print message, exit 1, no stack trace), and it has non-obvious branches worth locking in:

• the instanceof PakeError path;
• the cross-module duck-typed fallback (typeof error === 'object' && error.isUserError === true), which exists precisely because instances from a different module copy can fail instanceof;
• the error !== null guard (since typeof null === 'object').

Proposal

Add tests/unit/error.test.ts covering PakeError's flag/name and all isPakeError branches (PakeError instance, duck-typed object, plain Error, isUserError: false/{}, and null/undefined). Tests only. PR attached.