fix: replace stale 'bd mail' references with 'gt mail'

- docs/architecture.md: update mail routing explanation
- internal/witness/manager.go: fix actual bd mail calls to gt mail
- internal/cmd/mail.go: remove stale compatibility note
- internal/daemon/lifecycle.go: update comment

internal/daemon/lifecycle.go

@@ -32,7 +32,7 @@ func (d *Daemon) ProcessLifecycleRequests() {
 
 	output, err := cmd.Output()
 	if err != nil {
-		// bd mail might not be available or inbox empty
+		// gt mail might not be available or inbox empty
 		return
 	}