fix(shutdown): use KillSessionWithProcesses to prevent orphans

Address review feedback: revert KillSession back to KillSessionWithProcesses
in stopSession() to properly terminate all child processes.

KillSessionWithProcesses recursively finds and terminates descendant processes
with SIGTERM/SIGKILL, preventing orphaned Claude/node processes that can
survive tmux session kills.

The orphan detection in verifyShutdown() remains as a helpful warning but
shouldn't replace proper process termination.

internal/cmd/down.go

@@ -393,8 +393,8 @@ func stopSession(t *tmux.Tmux, sessionName string) (bool, error) {
 		time.Sleep(100 * time.Millisecond)
 	}
 
-	// Kill the session
-	return true, t.KillSession(sessionName)
+	// Kill the session (with explicit process termination to prevent orphans)
+	return true, t.KillSessionWithProcesses(sessionName)
 }
 
 // acquireShutdownLock prevents concurrent shutdowns.