fix: interpolate command and entrypoint environment

[saeidakbari]

Summary

• Resolve Compose variables in each list-form command and entrypoint token.
• Use the loaded project environment, preserving process-environment precedence.
• Add command and entrypoint regression coverage.

Verification

• swift test --filter EntrypointCommandTests (16 tests passed)
• Rebuilt with swift build -c release
• Ran the release binary against the Artifex migration service; the prior literal-variable URL parse failure no longer occurs.