make lint fails on shellcheck style and info warnings #167

Steps

make lint

Actual

shellcheck -e SC2008 gh-md-toc exits with code 1 on master (c386c7b, shellcheck 0.11.0):

• SC2006 (style): legacy backticks instead of $(...) - lines 170, 187, 246, 305-308, 371
• SC2181 (style): exit code checked indirectly with $? - line 89
• SC2016 (info): expressions in single quotes - lines 249, 258

Expected

make lint passes.

Notes

• CI (ci.yml) runs only make test, so the failure is not visible there.
• Lines 249 and 258 start the awk scripts, which are single-quoted on purpose so that
  $0 reaches awk unexpanded. SC2016 there likely needs a disable directive rather
  than a code change.