Add shellcheck

[jcbhmr]

This PR would...

• add a shellcheck https://www.shellcheck.net/ test

This would make sure that we do everything the "right way"! 😊

[jcbhmr]

./src/clone.sh:47:8: warning: Declare and assign separately to avoid masking return values. [SC2155]
./src/init.sh:47:8: warning: Declare and assign separately to avoid masking return values. [SC2155]

export GIT_DIR=$(mktemp -d)
# vs we want this:
export GIT_DIR && GIT_DIR=$(mktemp -d)

that way if mktemp fails, it bubbles instead of 'export' command returning zero and "this is fine, continue" even though the mktemp failed

[jcbhmr]

Merging this as I think this is a pretty safe "add checks to make better code" change.