fix(terminal): ignore invalid shell config from race condition

Reject shell paths with length <= 1 to prevent errors when user
input is accidentally sent as init message.

Assisted-by: MiniMax-M2.7 via Crush <crush@charm.land>

internal/api/terminal.go

@@ -105,6 +105,12 @@ func (s *Server) handleTerminalWS(w http.ResponseWriter, r *http.Request) {
 			}
 		}
 
+		// Ignore invalid shell paths (e.g., single characters from race condition)
+		if len(shell) <= 1 {
+			conn.WriteJSON(wsMessage{Type: "error", Data: "invalid shell config"})
+			return
+		}
+
 		if _, err := os.Stat(shell); err != nil {
 			conn.WriteJSON(wsMessage{Type: "error", Data: fmt.Sprintf("shell not found: %s", shell)})
 			return