Copilot Chat window 📎 file list spuriously includes DerivedData generated files

**Describe the bug**
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 notice that MANY built files in Xcode's DerivedData folder are spuriously visible in my GitHub Copilot Chat 📎 file list. I keep my DerivedData folder in the same folder as my Xcode project file for easier visibility, and the 📎 file list UI seems to have picked up all of these generated files!

To reproduce this, open Xcode Settings >> Location >> Derived Data; set Derived Data: to [Relative], and enter "DerivedData" in the textfield. Then build once, and see what appears in the 📎 file list. 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. Configure Xcode's Location preferences to place the DerivedData folder within the project/workspace directory. (See screenshot) Open Xcode Settings >> Location, set Derived Data: to [Relative], and enter "DerivedData" in the textfield.
2. Build your Xcode project.
3. From the macOS menubar open Editor >> GitHub Copilot >> Open Chat (having installed GitHub Copilot for Xcode, of course—NOT the independently developed "Copilot for Xcode").
4. Tap the 📎 button in the Chat window to open the file list and scroll through the available files.

**Results**
_Expected:_ Only source files from the project / workspace / file system appear in the file list.
_Actual:_ MANY files that are found in a local DerivedData folder pollute the file list in the Chat window.

**Impact**
Including generated files in the 📎 file list makes it harder for users to find the ones they want to share with GitHub Copilot.

**Screenshots**
1 - DerivedData Location setup
2 - spurious DerivedData files

**Logs**
N/A

**Additional context**
Excluding a local DerivedData folder is an easy fix, but there may be a need for a broader feature that lets users exclude unwanted files and folders from this list or include wanted files and folders in the list.