* feat(scripts): add shellspec installation to dfm
Add install-shellspec.sh script that clones shellspec to
~/.cache/shellspec and installs via make to ~/.local/bin.
Wire it into dfm install menu and the Tier 4 install-all pipeline.
* fix(scripts): address PR review feedback for shellspec installer
Add companion install-shellspec.md documentation file to match
codebase convention. Add --depth=1 to git pull for consistent
shallow clone behavior.
* fix(scripts): pin shellspec install to latest release tag
Use x-gh-get-latest-version to fetch the latest release tag from
GitHub and clone/checkout that specific version. Addresses supply
chain concern and normalizes --depth=1 style.
* docs(scripts): update shellspec docs to reflect release-tag pinning
