test: add bats tests for each bin script

[MekDrop]

Summary

• add ests/ Bats suite with coverage for every script in �in/
• add shared test helpers for temp directory and command stubbing
• add PR workflow (.github/workflows/test.yml) that installs Bats and runs �ats tests

Notes

• tests stub external tooling (like git and yq) where needed to keep behavior deterministic
• checkout.sh tests reflect the current branch-handling logic as implemented today

Testing

• unable to run Bats in this local Windows shell because /bin/bash is unavailable; tests are intended to run in CI