fix(cli): initialize zsh completion system before registration

[ian-pascoe]

Summary

• fix zsh completion script ordering so compinit -i runs before compdef _crosspack crosspack
• keep existing bash/fish/powershell completion loader behavior unchanged
• add a regression test that fails if zsh registration happens before compinit initialization

Test Plan

• rustup run stable cargo test -p crosspack-cli

[ian-pascoe]

@codex review thoroughly for bugs and security issues

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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