[feat] Make `@` file lookup fuzzier / greedier

Currently, searching for files to add to context can be frustrated by minor inaccuracies in the entered text - especially capitalization.

https://github.com/user-attachments/assets/c9fe31ea-e390-478e-af87-4f112354c8e9

By comparison, lookups in most IDEs are a little greedier - see Zed's response here:

<img width="539" height="361" alt="Image" src="https://github.com/user-attachments/assets/4ee83e5e-3e56-4635-abe5-015d6b227326" />

Speed is key, so I wouldn't advocate for any complex search strategy, but fuzzy and case-insensitive seem like immediate winners to me, especially the style of fuzziness that lets me leverage my knowledge of the codebase to very quickly surface specific files with short inputs. (the package.json of the db package in a monorepo here)

<img width="551" height="122" alt="Image" src="https://github.com/user-attachments/assets/bf73468c-e7f5-4662-bbff-785b77435378" />