Fix Debian package contents and add artifact test

[msander]

Summary

• keep the Debian README in the shared contents list with a deb-only packager flag so nfpm no longer drops the binary and unit when overrides are applied
• teach the packaging helpers and new deb artifact test to build from the repository root and assert the package ships the binary, config, unit, and tmpfiles entry
• document the additional packaging verification in docs/STATE.md so the roadmap reflects the stronger test coverage

Testing

• go test ./...

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https://chatgpt.com/codex/tasks/task_b_68cdb90930f08321aeb6681b0a442bb9