feat(scripts): add shellspec installation to dfm (#296)

* 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

scripts/install-shellspec.md

@@ -0,0 +1,16 @@
+# install-shellspec
+
+Installs [shellspec](https://github.com/shellspec/shellspec), a BDD-style
+testing framework for shell scripts.
+
+## Usage
+
+```bash
+scripts/install-shellspec.sh
+```
+
+The script resolves the latest release tag via `x-gh-get-latest-version`,
+clones shellspec to `~/.cache/shellspec` pinned to that tag, and runs
+`make install PREFIX=$HOME/.local`, placing the binary in `~/.local/bin/`.
+Re-running the script fetches and checks out the newest release tag
+before reinstalling.