What type of PR is this?

• bug

What this PR does / why we need it:

The completion command exposes each shell as its own subcommand (bash, zsh, fish, pwsh), introduced in #2279 (released in v3.9.1). The subcommands were generated by ranging over the shellCompletions map in buildCompletionCommand:

for shell, render := range shellCompletions {
    cmd.Commands = append(cmd.Commands, buildShellCompletionSubcommand(shell, render, appName))
}

Go randomizes map iteration order per process, so the order of these subcommands — and therefore the COMMANDS: section of completion --help — changed between runs. This breaks downstream projects that generate documentation from --help output and commit it via CI: the generated docs flip-flop depending on which run produced them, creating spurious diffs and noise commits. Real-world impact: suzuki-shunsuke/ghtkn#454

Changes:

• completion.go: add completionShells = []string{"bash", "zsh", "fish", "pwsh"}, an explicitly ordered list of the supported shells, and iterate it (looking the renderer up in shellCompletions) instead of ranging over the map. The map stays as the renderer registry; the slice fixes the display order. The order is now deterministic on every run.
• completion_test.go: add TestCompletionSubcommandOrder, which asserts the subcommand order is always bash, zsh, fish, pwsh and that every shell in shellCompletions is also present in completionShells (so the two cannot silently drift). TestCompletionShellRenderError and TestCompletionShellWriteError inject a custom shell into shellCompletions; they now also register it in completionShells (with restore) so the injected shell still becomes a subcommand.

Which issue(s) this PR fixes:

Fixes #2373

Special notes for your reviewer:

The display order is fixed to bash, zsh, fish, pwsh. If you would prefer alphabetical order instead, that is a one-line change to completionShells.

Testing

• go test ./... passes.
• The completion tests are stable across repeated go test invocations (separate processes), where the subcommand order previously varied.

Release Notes

Fix non-deterministic ordering of `completion` shell subcommands in help output; they now always appear as bash, zsh, fish, pwsh.