[CHORE] Cosmetic cleanup: justfile typo + stale Debian comments

[c-vigo]

Problem

• justfile:251 duplicated word: 'Cleaning Cleaning up lingering test containers…'.
• Several comments in tests/test_image.py, flake.nix, and assets/workspace/.devcontainer/scripts/post-create.sh still narrate 'the Debian image…' as historical context — harmless but searchable noise.

Recommendation

Single small cleanup commit; trim or link the Debian comments to #642. Do not over-engineer.

Context

Found during the state-of-the-repo review of #670 (epic #625). Low priority; can attach to #642.

Refs: #625