refactor(lint): fix shellcheck warnings

[ekalinin]

Summary

Fixes #167 together with #172: make lint passes once both are merged.

• SC2006 (9 places): legacy backticks replaced with $(...). None of them contain
  backslashes, so the result is the same.
• SC2016 (2 places): # shellcheck disable=SC2016 above each awkscript='{...}'
  assignment. The awk scripts are single-quoted on purpose so that $0 reaches awk
  unexpanded. The directive is set per assignment, not on the whole if block.

SC2181 is a real bug rather than a style issue and is fixed separately in #172.

No behavior changes.

Testing

• All 14 tests pass.
• --version output and the --insert footer (Added by: $(whoami), at: $(date)),
  which the tests do not cover, are the same as on master.
• shellcheck -e SC2008 gh-md-toc reports only SC2181, which fix(md2html): check curl exit status for network errors #172 resolves.