feat(kefka): parse scripts as bash
Pass syntax.Variant(syntax.LangBash) to mvdan.cc/sh's parser so kefka
accepts bash-flavored constructs (e.g. [[ ]], arrays, process
substitution) rather than the default POSIX subset.
Assisted-by: Claude Opus 4.7 via Claude Code
Signed-off-by: Xe Iaso <me@xeiaso.net>