Review found the build fallback could inherit a stale digest pin (compose build aborts), a slash-shaped/non-ghcr lock ref caused a doomed pull, env mode never persisted the pin (host restart used a nonexistent default tag), and the resolver test was unlinted/unrun in CI. Resolver now always leaves a coherent pin, validates ghcr.io/<owner>/<name>@sha256 refs only, exports the mode flag (drops the file-level shellcheck disable), and CI lints + runs the script unit tests.
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