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/cmd/mail.go

@@ -63,7 +63,7 @@ Message types:
   notification  - Informational (default)
   reply         - Response to message
 
-Priority levels (compatible with bd mail send):
+Priority levels:
   0 - urgent/critical
   1 - high
   2 - normal (default)