Add Docker HEALTHCHECK and mail-path /healthz liveness; env-doc regression
test; architecture.md and development.md; product.md and expanded security.md;
retire live specification.md to docs/archive/.
Co-Authored-By: Claude <claude-opus-5-thinking-high@noreply@anthropic.com>
Keeps implementation-plan.md focused on unresolved v1.0/v1.x questions;
inbound relay (Phase O1) and the domain-admin role now live in
docs/roadmap.md, cross-linked from progress.md.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
mix