ShellCheck rejects the runtime-selected platform environment

Symptom: Quality and macOS smoke fail with SC1090 in install/_lib.sh, while the Docker bootstrap suites pass.

Root cause: The platform environment path contains a runtime-selected $PLAT, so ShellCheck cannot resolve it. The Docker suite does not run the separate CI lint gate.

Confirm: Run shellcheck -S warning bootstrap.sh install/*.sh install/plat/*/.plat_env.sh home/dot_local/bin/executable_git-wt from the repo.

Fix: Mark only the dynamic import with source=/dev/null and lint every actual platform environment explicitly in both CI jobs. Keep the global lint severity unchanged; a passing bootstrap suite does not replace this check. The jobs now share tests/ci.sh; enable the push gate so the same checks validate the outgoing commit before Git publishes it.