Without ROOM_PASSWORD the guest password defaults to empty, and a client
sending a blank password is granted read+write access (can post). Every
other board's room_server env sets ROOM_PASSWORD; the ThinkNode M6 and M1
were the only two shipping an open room. Default them to "hello" to match
the rest of the tree; operators can still change it at runtime with
'set guest.password'.
The M6 is the only board whose simple_repeater env ships with MESH_DEBUG=1
and GPS_NMEA_DEBUG=1 enabled. MESH_DEBUG=1 adds a 5-second boot delay
(examples/simple_repeater/main.cpp) plus verbose per-packet serial prints on
the hot RX/TX path; GPS_NMEA_DEBUG=1 echoes every GPS UART character to the
serial console (MicroNMEALocationProvider.h). Every sibling repeater env
(M1, M3, t1000-e, RAK, Heltec) ships these off, and the M6 room_server env
in this same file already has them commented. Comment them out to match.
