[BUG] Diff panels not cleared when new suggestion is shown — stale diffs accumulate until window focus change

## Bug Description

After the latest update, sometimes diff panels shown in Neovim are not cleared when Claude Code proposes a new change. Previous diffs remain visible in the split panels and accumulate over time. The only way to dismiss them is to manually switch focus to the LazyVim tab.

## To Reproduce

Steps to reproduce the behavior:

1. Open Ghostty with a tab split in two panes: Claude Code on one side, LazyVim on the other.
2. Start the Claude Code integration in Neovim.
3. Ask Claude to make a change to a file — a diff is shown in the split panels.
4. Ask Claude to make a second (different) change to the same or another file.
5. Observe that the previous diff is still visible alongside the new one — diffs accumulate.

## Expected Behavior

When Claude Code proposes a new change, any previously displayed diff panels should be cleared/replaced, so that only the current suggestion is visible.

## Environment

- Neovim version: 0.11.6
- Claude Code CLI version: 2.1.70
- OS: macOS 26.3
- Plugin version: commit 432121f

## Error Messages

No explicit error messages — visual/rendering issue only.

## Additional Context

The issue seems to be related to focus/window state: switching focus to the LazyVim pane forces the stale diffs to disappear. This suggests the diff buffers or windows are not being properly closed/reset when a new suggestion is triggered.

新しい提案を処理し、diff バッファーまたはウィンドウを管理する Neovim Claude Code 統合パスから始めます。issue では具体的なファイルやテストは指定されていません。2 回連続して変更を加えて再現し、その後、現在の diff だけが表示され続け、古いパネルを消去するためにフォーカスを切り替える必要がなくなっていることを確認します。

索引モデルが issue の本文から書いたものです。