Fix shellcheck SC2068 (unquoted array expansions), SC2145 (mixed
string/array arguments), SC2328 (redirection in command
substitution), SC2173 (untrapable signals), and SC2148 (missing
shebang) errors across 14 script files.
Also configure scripts/Makefile with --severity=error so that only
errors (not warnings or notes) cause check failures. To be tightened
by follow-up commits.
Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL and pi.dev
Signed-off-by: Jan Lindemann <jan@janware.com>