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.

scripts/install-shellspec.md

@@ -0,0 +1,14 @@
+# 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 clones shellspec to `~/.cache/shellspec` and runs
+`make install PREFIX=$HOME/.local`, placing the binary in `~/.local/bin/`.
+Re-running the script pulls the latest version and reinstalls.