Eight names are exported as public API and documented nowhere

[jeremymanning]

Summary

Measured against clustrix.__all__ and every .rst, .ipynb and README.md under the repository:

53 exports, 8 appear nowhere in the docs

setup_environment              add_host_key        ProfileManager
setup_ssh_keys                 PackagedFile        create_modern_cluster_widget
                                                   display_modern_widget
                                                   show_widget

All 53 are importable and all have docstrings, so this is a reference gap rather than a broken surface. But a name in __all__ is public by declaration: it is what a user is invited to import, and four of these are things someone would plausibly reach for.

Which of these actually matter

• setup_ssh_keys and add_host_key are the friction points the SSH story is built around — generating a key and trusting a host are the first two things a new user does. docs/source/ssh_setup.rst covers the topic without naming either function.
• ProfileManager is the whole profile system behind the notebook widget, and the widget's own page does not mention it. A user who wants profiles from a script has no documented route.
• show_widget, display_modern_widget, create_modern_cluster_widget are three entry points to the same widget. If they are not all meant to be public, the fix is to shrink __all__ rather than to document all three.
• setup_environment and PackagedFile are plausibly internal. If so, they should come out of __all__.

Why this is worth recording

__all__ is the contract. Every name in it is either something a user should be able to find in the documentation, or something that should not be in __all__. Right now eight names are in neither state, and the two possible fixes point in opposite directions — so the decision has to be made per name rather than by adding eight reference stubs.

Definition of done

• Each of the eight is either documented where a user would look for it, or removed from __all__
• A test asserts the invariant, so a new export cannot arrive undocumented — the measurement above is about ten lines
• If the three widget entry points stay, the documentation says which one to reach for and why the others exist