resolveMailuConfig now defaults to the real bbs mail stack: address domain
bbs.profullstack.com, mail host mail.profullstack.com, SMTP 465 implicit TLS
(bbs refuses 587), IMAPS 993. Also fixes a latent mismatch where the address
domain (bbs) and the transport's from-domain check (mail) diverged with no env
set. Verified live end-to-end with only AGENTMAIL_USER/PASS supplied.
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>