Cannot select non-code files (.md, .txt) to include in chat window via 📎 file list button

In the Chat window for GitHub Copilot for Xcode (v0.31.0), non-code files, specifically markdown and text files (.md and .txt files) are not listed in the file list when I tap the 📎 button.

It is therefore impossible for Copilot to access the contents of a `README.md` or `task_list.txt` file directly within the Copilot UI. Thanks for taking a look.

**Versions**
- Copilot for Xcode: 0.31.0
- Xcode: 16.2 (16C5032a)
- macOS: Sequoia 15.3.1 (24D70)

**Steps to reproduce**
1. Open an Xcode project that contains a `.txt` or `.md` file on macOS.
2. Select GitHub Copilot >> Open Chat from the macOS menubar (having installed GitHub Copilot for Xcode, of course—NOT the independently developed "Copilot for Xcode").
3. Attempt to select one of these non-code files (e.g. `README.md` or `task_list.txt`) from the 📎 file list to provide it as context in Copilot chat.

**Results**
_Expected:_ Non-code files like `README.md` and `task_list.txt` are listed in the 📎 file list in the Copilot Chat window.
_Actual:_ Non-code files do not appear in the 📎 file list and thus cannot be shared as context in Copilot Chat.

**Impact**
This limits users' ability to use GitHub Copilot for Xcode to maintain task lists, README, license files, release notes, and other non-code documents.

**Workaround**
Rename `README.md` to `README.md.swift` and the file now appears in the 📎 file list.

**Screenshots**
0 - project layout
1 - .md file missing from file list
2 - workaround by renaming files

**Logs**
N/A

**Additional context**
There is a problem in which `DerivedData` files are spuriously included in the 📎 file list menu (if the `DerivedData` is located in the project directory). I'll file a separate ticket for that, but these could probably both be addressed together.

ps. I'm open to full-time or contract work and would love to contribute if there is an opportunity. https://www.linkedin.com/in/billfeth Thanks!