Agent Host file tools do not support extension-registered filesystems

### Description

Agent Host file operation tools such as reading and editing require native operating-system paths and cannot access resources provided by extension-registered filesystem providers.

These resources work normally through VS Code’s filesystem and editing APIs. Extensions can provide custom tools as a workaround, but this requires each filesystem extension to reimplement standard reading and editing operations.

**Expected**

Agent Host providers should use VS Code’s filesystem and editing abstractions for non-native workspace URIs, while continuing to use native tools for local files.

**Actual**

Agent Host file tools cannot read or edit extension-backed workspace resources, even though VS Code can access them.

This affects Codex Agent Host and could affect other Agent Host providers that assume every workspace resource has a native path.

Related startup failure: #335034.

### Issue Details

Item|Value
---|---
Issue Category|Bug
Target|Visual Studio Code - Insiders
VS Code Version|Code - Insiders 1.137.0-insider (773da685a20213ce5755a80f523101669b145702, 2026-09-07T22:35:28Z)
OS Version|Windows_NT x64 10.0.26200

### Additional Issue Data

[issue-data.md](https://github.com/user-attachments/files/31952108/issue-data.md)

### Attachments