I implemented file referencing feature mentioned in issue #1591
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## Summary by cubic
Adds support for referencing app files in chat using @file:<path>. The
input now autocompletes files, and the backend validates and surfaces
referenced files in the chat context as read-only.
- **New Features**
- Chat input autocompletes @ mentions for apps, prompts, and files.
- Recognizes @file:<path> and converts it to a structured mention on
submit.
- Backend parses @file mentions, checks file existence, and adds a
“Referenced Files” section to the system message.
- New get-app-files IPC handler and useAppFiles hook to load file paths
for the selected app (appFilesAtom added).
- e2e test for mentioning a file and capturing server dump.
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