Add ring completions for bash, zsh and fish

[Shine-neko]

ring completions <bash|zsh|fish> prints a shell completion script on stdout, so <TAB> completes commands, subcommands and flags.

ring completions zsh > ~/.zsh/completions/_ring

ring deplo<TAB>             -> deployment
ring deployment <TAB>       -> list inspect delete logs events metrics health-checks
ring deployment logs -<TAB> -> -f -c --follow --tail --since --container

• The script is generated from the CLI tree itself (extracted into build_cli()), so completions can never drift from the real commands: a new subcommand is completable with no extra work.
• completions is dispatched before the config is loaded, so generating a script works on a machine that has never run ring init.
• Restricted to the three shells the docs cover. clap_complete also knows elvish and powershell; accepting values the docs don't describe would put --help and the reference out of sync.
• Completion covers command names and flags, not values: deployment IDs would require calling the API from the shell's completion hook.

Verified by loading the generated scripts into real shells, not just by generating them: the bash examples above are actual COMPREPLY output, and the zsh script loads via fpath + compinit. Syntax checked with bash -n / zsh -n. Fish is generated and covered by unit tests, but could not be loaded here (fish is not installed on the dev machine).

The second commit fixes three pre-existing clippy errors (apply.rs, container.rs, untouched since March) that -D warnings turns into failures on a newer clippy. They were blocking the pre-push hook and are unrelated to this feature.