Add shell completions for Killgrave CLI

[joanlopez]

Based on the most recent changes present on the branch v0.5.0, it seems Killgrave is introducing the use of Cobra as the library used to build and run the CLI commands.

Thus would be good to also add the shell (Bash, Zsh, fish, PowerShell) completions supported by the library.
More information here.

[MayukhSobo]

@joanlopez The document link is not working!

[joanlopez]

Looks like now it's under https://github.com/spf13/cobra/blob/main/site/content/completions/_index.md
Are you planning to work on this? @MayukhSobo

[MayukhSobo]

@joanlopez Yes I would be happy to work on it. Can you assign this to me?

[MayukhSobo]

Hi @joanlopez, I literally forgot about this issue, and today, while exploring my GitHub open issues, I see it's still not solved and is still assigned to me.

I've spent some time investigating and here's where things stand:

Current state: Killgrave uses Cobra v1.8.1, which does NOT auto-generate a completion subcommand when the CLI has no subcommands (which is the case for Killgrave). So killgrave completion bash currently doesn't work — it just tries to start the server and fails.

Proposed approach:

1. Upgrade Cobra to v1.10.2 (latest). There's already an unmerged Dependabot PR for v1.9.1. Starting from v1.9.1, Cobra automatically provides a hidden but functional completion subcommand. This alone would make killgrave completion bash/zsh/fish/powershell work.
2. Add an explicit completion subcommand so it's visible in killgrave --help — since this is a setup command users need to run once to install completions into their shell, they should be able to discover it.
3. Register smart completions for flag values — e.g. --proxy-mode suggesting all|missing|none, --config filtering to .yml/.yaml files, --imposters completing only directories.

I'm happy to pick this up if you're okay with the approach. Would you prefer the Cobra upgrade in the same PR or separate?

This time I shall not forget 😄