Layout consistency: everything ckn-bw deploys to the host now lives
under deploy/. ckn-bw's install_left4me_scripts copy-action goes away
in lockstep with this commit and is replaced by target-side symlinks.
Also updates all path references in docs, tests (conftest.py parents[]
depth, test_overlay_helper.py HELPER_SOURCE), and deploy/README.md.
Part of 2026-05-15-deployment-responsibility-design.md migration step 4.
