Complete while typing with prompt_toolkit

Replace readline with a prompt_toolkit session: the completion menu opens
as you type, history suggestions show in gray (accept with Right), and
history lives in .cache/history. Enter on a highlighted candidate accepts
it instead of submitting the line. Ctrl+C now exits cleanly.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

requirements.txt

@@ -1,3 +1,4 @@
 torch
 transformers
 huggingface_hub
+prompt_toolkit