ubuntu-latest ships shellcheck 0.9.0, which warns that ash scripts are checked as dash; 0.10 (local) does not. shell=dash is the accurate dialect for busybox ash. Verified clean under BOTH versions; boot smoke re-run. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_018HUayid7W5w7jBdb9Rrj1K