One bus: the AMQP transport is gone from the host

The mesh runs on the seat's bus alone (novox/hq ADR 0131, design 28 task 5.5). The host's old
dialling and enrolment paths are deleted with the switch that chose between them; a membership or
a token naming another bus is refused before anything is sent, rather than dialled on a transport
that no longer exists.

internal/link/run.go

@@ -30,9 +30,8 @@ type Membership struct {
 	Fingerprint string
 	Password    string
 	Signer      ed25519.PublicKey
-	// Transport is which bus this node speaks (hearing.go). Empty is the one the mesh runs on
-	// today, which is every node until the rollout — so a membership recorded before any of this
-	// existed reads as correct rather than as unset.
+	// Transport is which bus this membership was minted for (hearing.go): the mesh's own, and a
+	// membership that names another is one this host cannot dial with.
 	Transport string
 }