The forced line break (Ctrl+Enter) never worked: the target terminal does not
distinguish any modifier+Enter from plain Enter — all of Ctrl/Alt/Shift+Enter
arrive as key='enter' character='\r' (verified via a one-off key probe). With
Enter bound to submit, any modifier+Enter newline binding would collide with
submit and is impossible to distinguish in software.

Remove the Ctrl+Enter branch from _PromptInput._on_key. The input stays
multi-line via soft-wrap (TextArea soft_wrap=True) and height: auto growth;
Enter still submits. Placeholder and module docstring updated to drop the
Ctrl+Enter mention and note why a newline key is not wired (revisit later).

Tests: removed test_ctrl_enter_inserts_newline_without_submitting; kept the
multiline-submit and input-grows tests. 576 passed, 1 skipped.

Co-Authored-By: Claude <noreply@anthropic.com>