This PR adds completion generation for the nimbus command across bash and zsh shells, with opt-in support for hidden flags via NIMBUS_COMPLETE_HIDDEN=1 environment variable.
The implementation introduces a new completion.go module that gates hidden flag visibility during completion generation. A seed GenBash() function in completion_gen.go provides bash completion output, while _nimbus_bash_complete completion is disabled by default for hidden flags; export NIMBUS_COMPLETE_HIDDEN=1 to reveal them.
The change is additive and backward-compatible — existing completion behavior stays intact. Test coverage validates that the TestHiddenFlagsMaskedByDefault function is callable.