fix: export env vars in bash/zsh completion hooks

[LeTamanoir]

Summary

• The bash and zsh completion registration scripts set COMPLETE and _COMPLETE_INDEX as shell variables inside $(...) subshells, but never export them
• The CLI binary checks process.env.COMPLETE to enter completion mode - since the vars aren't exported, it never sees them
• The binary falls through to normal arg parsing, hits the -- separator, and outputs an error that the shell interprets as completion candidates
• Fish and nushell are unaffected (they use inline prefix syntax like COMPLETE=fish cmd ...)

Reproduction

# This is what the generated zsh hook does internally:
result=$(
    COMPLETE="zsh"
    env | grep COMPLETE  # nothing - not in env
)

# Fix: export makes it visible to child processes
result=$(
    export COMPLETE="zsh"
    env | grep COMPLETE  # COMPLETE=zsh
)

[LeTamanoir]

Sorry somehow I opened a PR about this but mistakenly closed it, gonna try to not blow up this one 😁