add shell completions (bash/zsh/fish/PowerShell)

[bmertens-datum]

Summary

datumctl has no shell completions today. Users tab-completing commands,
subcommands, flags, and resource type names get nothing. Cobra has built-in
completion support — the bulk of the work is wiring it up and adding dynamic
completion for resource type arguments.

Note: This is a human UX feature. AI tools and automated pipelines
construct commands programmatically and do not benefit from shell completions.
The target audience is developers typing datumctl interactively in a terminal.

What should complete

Input	Completes to
datumctl <TAB>	subcommands (get, apply, auth, ai, …)
datumctl get <TAB>	resource types fetched live from api-resources (e.g. projects, dnszones, domains)
datumctl describe <TAB>	same as above
datumctl delete <TAB>	same as above
datumctl edit <TAB>	same as above
datumctl explain <TAB>	same as above
datumctl auth <TAB>	auth subcommands
datumctl --<TAB>	flags for the current command

Implementation notes

Cobra's completion command (cmd.GenBashCompletion, GenZshCompletion, etc.)
handles static command/flag completion out of the box — just add the completion
command to root.

Dynamic resource type completion requires registering a ValidArgsFunction on
get, describe, delete, edit, and explain that calls the API and returns
resource type names. This is the same data datumctl api-resources returns; the
completion function can reuse that path.

The completion command should be hidden from the main help group (same pattern as
generate-cli-docs) since it is a setup utility, not an everyday command.

Acceptance criteria

• datumctl completion bash / zsh / fish / powershell outputs a valid completion script
• datumctl get <TAB> completes with live resource types
• datumctl describe/delete/edit/explain <TAB> get the same dynamic completions
• Flags complete on all commands

[gianarb]

@bmertens-datum I see a completion command that looks like what you are describing. Did you check it?

[bmertens-datum]

@gianarb It's does not work for me? Does it work for you? Maybe a windows issue?