Feature Request: use @ to define explicit file/path to be added into conext as available in Q IDE

Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my

Operating system
macOS 15.5.0 (24F74)

Expected behaviour
Amazon Q CLI should allow to specify a file path as available per Q IDE and other CLI assistants (Gemini/Claude), "@ for file paths" and it even allows to show a drop down of files in case there are duplicates.

This feature would be particularly valuable for:
- The q developer @file command allows you to specify a particular file as context for Amazon Q Developer, enabling it to provide more focused and relevant responses based on the contents of that file.
- give instructions to work with specific file(s)
- speed up development with drop-down selection if file name is not directly visible in the terminal

Actual behaviour
- @ seems to be reserved for @ [...args]

Inizia tracciando il parsing esistente di @ e il punto di ingresso della gestione del contesto nella CLI. Definisci come i percorsi dei file coesistono con i nomi dei prompt, come vengono selezionati i nomi di file duplicati e come i file espliciti compaiono nella richiesta; il lavoro è completato quando questi casi funzionano senza interrompere i prompt @ esistenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.