If a custom command's "context" field contains a context name that
doesn't exist, lazygit panics when building the keybindings. This could
happen either because of a typo, or because a context is removed or
renamed in a later version. Prevent the panic by validating those names
at config load time, and rejecting the config as invalid there, like we
do for other config errors.
The gui package owns the list, but can't be imported from here, so it is
mirrored and a test over there ensures the copies stay in sync.
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
