feat(completions): add bash and zsh completions

[CheapFuck]

Closes #5.

Adds shell completion scripts for bash and zsh.

What

• New user/local/share/towel/completions/towel.bash and .../_towel.
  They are installed to ~/.local/share/towel/completions/ automatically by the existing towel_install_from_dir copy step (the whole user/local/share/towel tree is copied during ./install).
• Completions cover:
  • subcommands: create enter exec export install remove update
  • top-level options: -h/--help, -v/--version
  • per-subcommand flags (e.g. update --check/--apply/--no-auto-check/--auto-check/-y/--yes/--quiet, install -y/--yes, remove -y/--yes/-h/--help)
  • exec completes the command to run inside the container
• Package-name positions (install/export) are intentionally left to default completion to avoid slow container/dnf queries.
• README gains a Shell completions section explaining how to enable them for bash (source the file) and zsh (add the dir to fpath before compinit).

Notes

• The completion files live under completions/, not bin/, so they are outside the CI shellcheck/shfmt glob (bin/towel and bin/towel-*). This is deliberate: the zsh #compdef file is not POSIX/bash and would not pass plain shellcheck. The existing CI set still passes (verified locally with shellcheck -x and shfmt -d).
• The bash completion was exercised locally (top-level commands, partial matches, per-subcommand flags, and empty package positions all behave as expected).
• I left the installer enabling of completions as a manual, documented step rather than auto-editing .bashrc/.zshrc (especially compinit), to avoid intruding on user shell config. Happy to wire it into ./install if you prefer.