Subscriber validation accepts malformed email addresses

Reproduction

Run:

pnpm install
pnpm test:subscribers

Expected

validateSubscriberEmail("reader@") should return { ok: false, error: "Enter a valid email address." } because the address has no domain.

Actual

The function returns { ok: true, email: "reader@" }.

Notes

This is an intentional repro issue for testing a sandboxed HarnessAgent triage workflow.