Settings overrides (e.g. local project settings, server settings) can no
longer change `disable_ai` to `false` if it was `true`; they can only
change it to `true`. In other words, settings can only cause AI to be
*more* disabled, they can't undo the user's preference for no AI (or the
project's requirement not to use AI).

Release Notes:

- Settings overrides (such as local project settings) can now only
override `disable_ai` to become `true`; they can no longer cause
otherwise-disabled AI to become re-enabled.

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Co-authored-by: Assistant <assistant@anthropic.com>
Co-authored-by: David Kleingeld <git@davidsk.dev>