Problem:
In aec3d7915c Vim changed prompt-buffers
to respect 'modified' so the termdebug plugin can "control closing the
window". But for most use-cases (REPL, shell, AI "chat", …),
prompt-buffers are in practice always "modified", and no way to "save"
them, so *implicitly* setting 'modified' is noisy and annoying.
Solution:
Don't implicitly set 'modified' when a prompt-buffer is updated.
Plugins/users can still explicitly set 'modified', which will then
trigger the "E37: No write since last change" warning.
689a149b08