feat: shell completion

[foreignmeloman]

I could not find any way to get or generate bash completion i.e. source <(k9s completion bash). I use k9s with CLI options quite often and having a proper completion is a must for me. Did not find any mention in the docs either.

[nklmilojevic]

Agreed. Shell completion should be available for CLI options and subcommands. We will add sofka completion <shell> for Bash, Zsh, Fish, Elvish, and PowerShell, with setup instructions for each shell. The scripts will come from the CLI definitions so they stay in sync, and generation will not need a cluster connection. I will track this in an issue and link the pull request here.

[nklmilojevic]

Tracked in #621 and implemented in #622. The pull request adds Bash, Zsh, Fish, Elvish, and PowerShell scripts, with setup instructions for each shell. Local checks passed, including all pre-commit hooks with tests run in sequence.