The final instructions printed commands with plain echo, so the colour
variables appeared as literal "\033[0;34m" text. Use echo -e for those
lines, as the rest of the script already does.
Make diagnose_bt.sh executable like the other scripts, so it runs without
the separate chmod step.
Resolve the shellcheck warnings in install.sh: declare and assign
separately (SC2155), drop an unused loop variable (SC2034) and a useless
cat (SC2002), and mark the ERR trap handler as indirectly invoked
(SC2317). No behaviour change.
