Fix zsh completion initialization

[esnssr]

Summary

this issues has been annoying me for a while, and i thought i didn't configure it correctly, hopefully this fixes the issue for other people, it works for me.

Changes

• initialize zsh bash completion support before registering the wut completion handler
• guard compinit so existing zsh completion registrations are not reset
• add a regression test for the generated init script ordering

Verification

• go test -count=1 ./...
• go build -o /private/tmp/wut-fork-pr-test ./cmd/wut
• zsh and bash init smoke checks