Fix stale default configuration after removing a saved config

[const-cloudinary]

__default__ could name a config that no longer exists, which suppressed the auto-default rule on the next save.

• load_config drops a __default__ naming no saved config; stale keys clear on the next write, since every mutation rebuilds the file from a load_config result.
• Add remove_named_config, counterpart to save_named_config: one atomic write that deletes the config and re-points the default. Used by logout and config -rm.
• Promote a lone survivor to default when the removed config held it, unless the environment configures Cloudinary - a stored default outranks the environment, so promoting would take precedence from CLOUDINARY_URL. _is_sole_usable_config holds that rule for the save and remove paths.
• cld logout with a single saved login confirms it instead of showing a one-item menu.

Brief Summary of Changes

What does this PR address?

• GitHub issue (Add reference - #XX)
• Refactoring
• New feature
• Bug fix
• Adds more tests

Are tests included?

• Yes
• No

Reviewer, please note:

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I ran the full test suite before pushing the changes and all the tests pass.