Desktop app: allow attaching files (not just images) to a prompt

Feature Description

Let the prompt composer attach arbitrary files, not only images.

Drag-and-drop and a file picker should accept .md, .txt, .json, .csv, .log, and source files. Text files get inlined into the message context with a token estimate; binaries and images continue through the existing image path. Show a chip per attachment with a remove action, and enforce a size cap.

Use Case

Right now I can only attach images. To get a .md file into context I have to describe its location inside the prompt ("the file is at x, go read it") and rely on the agent to open the right path — indirect, slow, and it fails when the agent guesses wrong. Attaching the file directly is more reliable and cheaper in tokens than narrating a path.

Additional Context

macOS 26.6.2 (build 25G83), arm64. Desktop app 0.1.35 (bundle ai.commandcode.desktop).

The two prose fields on this form (Feature Description, Use Case) have no id, so the app's prefilled issue URL cannot fill them — they are paste-only. Worth an id if attachment/prefill is ever wanted.

How important is this to you?

Important for my workflow