Shell completion support

Shell completions turn mytool dep<TAB> into mytool deploy and mytool deploy --en<TAB> into mytool deploy --environment. Power users depend on them; new users discover commands through them. A CLI with twelve subcommands and no completion support forces users to consult --help for every invocation. Completion scripts are cheap to generate (one line in cobra, one crate in clap, built into oclif) and disproportionately improve daily use.

Add a completions subcommand that emits bash/zsh/fish scripts to stdout, and document the install one-liner in README. For cobra in cmd/completion.go:

var completionCmd = &cobra.Command{
    Use: "completion [bash|zsh|fish]",
    RunE: func(cmd *cobra.Command, args []string) error {
        return rootCmd.GenBashCompletion(os.Stdout)
    },
}

Users install with mytool completion bash > /etc/bash_completion.d/mytool.