feat: adding shell completions

[ehfeng]

No description provided.

[tembo]

Overall this looks solid and keeps the interface simple.

Main things I called out:

• Prefer cmd.OutOrStdout() over os.Stdout so completion generation respects Cobra output configuration (helps with wrappers/tests).
• Return an explicit error in the switch default even though arg validation currently prevents it; this keeps behavior safer if validation changes.
• Tweak the Linux bash install snippet to use sudo tee so the /etc/... redirect is actually copy/paste-able.

[tembo]

Nice addition. Since the RunE below can use cmd.OutOrStdout(), you can drop the direct os.Stdout dependency and keep output redirection/testability consistent with Cobra.

Suggested change

	import (
	"os"
	"github.com/spf13/cobra"
	)
	import (
	"fmt"
	"github.com/spf13/cobra"
	)

[tembo]

Consider writing to cmd.OutOrStdout() instead of os.Stdout so this respects cmd.SetOut(...) (useful for tests/wrappers). Also, even though OnlyValidArgs should prevent this, returning an explicit error on unexpected values makes the function safer if the arg validation ever changes.

Suggested change

	RunE: func(cmd *cobra.Command, args []string) error {
	switch args[0] {
	case "bash":
	return cmd.Root().GenBashCompletion(os.Stdout)
	case "zsh":
	return cmd.Root().GenZshCompletion(os.Stdout)
	case "fish":
	return cmd.Root().GenFishCompletion(os.Stdout, true)
	}
	return nil
	},
	RunE: func(cmd *cobra.Command, args []string) error {
	out := cmd.OutOrStdout()
	switch args[0] {
	case "bash":
	return cmd.Root().GenBashCompletion(out)
	case "zsh":
	return cmd.Root().GenZshCompletion(out)
	case "fish":
	return cmd.Root().GenFishCompletion(out, true)
	default:
	return fmt.Errorf("unsupported shell: %q", args[0])
	}
	},

[tembo]

Minor docs nit: the Linux > /etc/... redirection will fail unless the whole redirect runs as root. Swapping to sudo tee tends to make this copy/paste-able.

Suggested change

	# Linux:
	$ kernel completion bash > /etc/bash_completion.d/kernel
	# macOS:
	$ kernel completion bash > $(brew --prefix)/etc/bash_completion.d/kernel
	# Linux:
	$ kernel completion bash | sudo tee /etc/bash_completion.d/kernel > /dev/null
	# macOS:
	$ kernel completion bash > $(brew --prefix)/etc/bash_completion.d/kernel

[ehfeng]

This already exists