Fix mv consistency and run bash comparisons in CI

[ivarvong]

What changed

• Make mv use filesystem-level move so it preserves metadata (mode) and handles directories/symlinks.
• Treat mv src dest/ as moving into the directory (basename join).
• Prevent crashes when moving a directory onto an existing file (return an error instead).
• Add new mv coverage, including bash-comparison tests.

CI

• Add a Linux-only job that runs mix test --only bash_comparison so bash-comparison coverage is enforced in CI.

Test plan

• mix compile --warnings-as-errors
• mix test --exclude bash_comparison
• mix test --only bash_comparison (runs in CI on Linux)