Extract the ssh debug commands into a transport neutral commands package (#1958)

Migrate commands from ssh into a transport neutral package

 - add golden tests validating that output remains unchanged
 - move commands to commands.go

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

control.go

@@ -11,6 +11,7 @@ import (
 	"syscall"
 
 	"github.com/slackhq/nebula/cert"
+	"github.com/slackhq/nebula/commands"
 	"github.com/slackhq/nebula/header"
 	"github.com/slackhq/nebula/overlay"
 )
@@ -47,6 +48,7 @@ type Control struct {
 
 	f                      *Interface
 	l                      *slog.Logger
+	commands               *commands.Registry
 	ctx                    context.Context
 	cancel                 context.CancelFunc
 	sshStart               func()
@@ -372,6 +374,10 @@ func (c *Control) CloseAllTunnels(excludeLighthouses bool) (closed int) {
 	return
 }
 
+func (c *Control) Commands() *commands.Registry {
+	return c.commands
+}
+
 func (c *Control) Device() overlay.Device {
 	return c.f.inside
 }