Custom agents not appearing in dropdown unless agent files are open in editor

Reproduce the issue with a valid .github/agents/my-custom-agent.agent.md file, then compare the agent dropdown before and after opening the file in Xcode. Trace how custom agents are discovered and loaded when Copilot Chat opens, and consider the reported expected behavior complete when agents remain available after the file is closed.

Description
Custom agents defined in .github/agents/ are not appearing in the Copilot chat agent dropdown in Xcode unless the agent file itself is currently open in a window/editor tab.
Expected Behavior
Custom agents should be automatically discovered and loaded from the .github/agents/ directory when:

Opening Xcode with a project
Opening Copilot Chat
Creating a new custom agent file

This is how custom agents work in VS Code - they appear in the dropdown automatically without requiring the files to be open.
Actual Behavior
Custom agents only appear in the Copilot chat dropdown when their corresponding .agent.md files are open in the Xcode editor. Once the file is closed, the agent disappears from the dropdown.
Steps to Reproduce

Create a custom agent file in .github/agents/ (e.g., my-custom-agent.agent.md)
Add valid YAML frontmatter and agent content:

---
name: my-custom-agent
description: Test custom agent
---
You are a helpful assistant.

Save the file and close it
Open Copilot Chat in Xcode
Click the agent dropdown at the bottom of the chat view
Result: The custom agent does NOT appear in the list
Now open the my-custom-agent.agent.md file in Xcode (in any window/tab)
Check the agent dropdown again
Result: The custom agent NOW appears in the dropdown

Environment

Copilot for Xcode version: [latest as of January 2026] (0.46.159)
Xcode version: 26.2 (17C52)
macOS version: 26.2 (25C56)
GitHub Copilot subscription: [Pro/Business/Enterprise]

Additional Context

The custom agent files are in the correct location (.github/agents/)
Files use the correct .agent.md extension
The .github folder is NOT ignored by .gitignore
The same custom agents work perfectly in VS Code without needing to open the files
No target property is set (or it's set to allow all IDEs)
This behavior is consistent across multiple custom agents

Verification Checklist
I have verified the following:

✅ Agent files are in .github/agents/ directory
✅ Files use .agent.md extension
✅ YAML frontmatter is valid
✅ .github folder is not git-ignored
✅ Latest version of Copilot for Xcode is installed
✅ Same agents work in VS Code
✅ No target: vscode property restricting the agent to VS Code only

Impact
This bug requires developers to keep all custom agent files open in editor tabs to use them, which:

Clutters the workspace
Defeats the purpose of having reusable, discoverable agents
Creates inconsistent behavior compared to VS Code
Makes custom agents essentially unusable in Xcode workflows

Suggested Fix
Copilot for Xcode should implement proper file system watching/indexing to automatically discover and load custom agents from .github/agents/ without requiring the files to be open in the editor, matching the behavior in VS Code and other supported IDEs.