feat: add headless mode support for SSH -N connections

- use s.lifecycle.GetConnection().Wait() to block until SSH connection closes
- Prevent premature session closure in headless mode

In headless mode (ssh -N), there's no channel interaction to block on,
so the session would immediately return and close. Now blocking on
conn.Wait() keeps the session alive until the client disconnects.

types/types.go

@@ -8,6 +8,13 @@ const (
 	SETUP        Status = "SETUP"
 )
 
+type Mode string
+
+const (
+	INTERACTIVE Mode = "INTERACTIVE"
+	HEADLESS    Mode = "HEADLESS"
+)
+
 type TunnelType string
 
 const (