`@mention`ed files in assistant2 now get replaced by the full path of
the file in what gets sent to the model, while rendering visually as
just the filename (in a crease, so they can only be selected/deleted as
a whole unit, not character by character).


https://github.com/user-attachments/assets/a5867a93-d656-4a17-aced-58424c6e8cf6

Release Notes:

- N/A

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Co-authored-by: João Marcos <joao@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>