cli: Add shell completion generation command

[tnk4on]

Summary

Add a hidden bootc completion <shell> subcommand that generates shell completion scripts for bash, zsh, and fish.

Motivation

This enables distributions (e.g., Fedora, CentOS Stream, RHEL) to generate and package shell completions during RPM/deb builds by calling:

bootc completion bash > /usr/share/bash-completion/completions/bootc
bootc completion zsh > /usr/share/zsh/site-functions/_bootc
bootc completion fish > /usr/share/fish/vendor_completions.d/bootc.fish

Implementation Details

• Added CompletionShell enum with Bash, Zsh, and Fish variants
• The completion scripts only include visible (non-hidden) subcommands intended for end users
• Completed subcommands: edit, image, install, status, switch, update, upgrade, usroverlay
• Uses clap::CommandFactory to introspect the CLI structure
• Added clap_complete as a dev-dependency

Testing

• Tested on Fedora container with cargo build
• Tested RPM build on RHEL10-compatible environment
• Verified completion works correctly in bash, zsh, and fish shells

[gemini-code-assist]

Code Review

This pull request adds a bootc completion command to generate shell completion scripts. The implementation is well-intentioned but manually generates the scripts, which is brittle and incomplete. I've provided feedback to use the clap_complete crate for a more robust and maintainable solution, which also aligns the implementation with the existing tests. I've also noted that clap_complete should be a regular dependency, not a dev-dependency, and pointed out some code duplication in the new tests.

[henrywang]

Could you please sign off commit which is required by DCO test and make a rebase to fix fedora-42 failure? Thank you and Happy New Year.

[cgwalters]

Also, thanks so much for working on this!!

[tnk4on]

I've made a few updates before pushing:

1. Local validation: Ran validation tests locally and fixed any issues before pushing to ensure the code is ready for review.

2. Focused changes: Removed unrelated file changes that were accidentally included. The PR now focuses only on the shell completion feature.

[cgwalters]

This enables distributions (e.g., Fedora, CentOS Stream, RHEL) to generate and package shell completions during RPM/deb builds by calling:

Also we do have a spec file here...so it'd likely be useful to actually do that change in it.

It looks like at least podman ships bash and fish completions by default. Also they ship a man page man podman-completion, which might make sense here.

---

Also I tested this out, it seems to work well but one thing I noticed I don't like is that it shows our hide=true subcommands. Digging in there's a ton of related issues/PRs in clap, of which clap-rs/clap#5283 is most relevant.