Lightweight File Editing Tab with Next Edit Suggestions (NES) for Quick Solution Verification

### Feature summary

Ability to perform lightweight file edits (e.g., tweaking config or .env files) directly within a dedicated tab in the GitHub Copilot app with Next Edit Suggestions (NES) enabled, allowing quick manual verification of suggested solutions without context switching.

### What problem are you trying to solve?

When verifying code changes proposed by GitHub Copilot, we often need to make small adjustments to settings, environment variables, or configuration files to manually test and verify if the proposed solution is correct.

Currently, having to switch out of the Copilot app into a full IDE just to tweak a single key in a .env or config file breaks the feedback loop and introduces friction during testing.

### Proposed solution

Introduce a lightweight, single-file editing tab within the GitHub Copilot app powered by Next Edit Suggestions (NES).

Key capabilities:
* Dedicated File Edit Tab: View and tweak setup, .env, .yaml, or .json files directly in the app.
* NES Integration: Enable Next Edit Suggestions within the edit tab to rapidly complete repetitive config adjustments, parameters, or environment keys.
* Rapid Verification: Adjust settings and immediately verify Copilot-suggested solutions in real-time.

Scope & Boundaries (Non-Goals):
To ensure this does not conflict with full-fledged IDEs like VS Code:
* In Scope: Lightweight single-file inline text edits and NES-driven completion specifically intended for rapid manual verification. Restrict file editing to a single tab to reinforce this behaviour.
* Out of Scope: Multi-file tree navigation, extension ecosystems, language server protocol (LSP) diagnostics, or rich debugging suites.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

この issue では、ファイル、テスト、既存のエントリーポイントが指定されていません。まず、アプリの現在の file-editing と Next Edit Suggestions のエントリーポイントを見つけ、次にそれらの機能を、示されている single-tab のスコープおよび非目標と比較してください。完了の条件は、軽量な single-file edits、NES のサポート、そして multi-file navigation、LSP diagnostics、リッチなデバッグなしでの迅速な検証です。

索引モデルが issue の本文から書いたものです。