feat(mcp-server): screenshot attached to create_new_diagram and edit_diagram results (#983)

* feat(mcp-server): optional screenshot attached to create_new_diagram and edit_diagram results

Both write tools take an optional boolean `screenshot`; the default comes
from DRAWIO_AUTO_SCREENSHOT. The body of screenshot_diagram moved into
captureScreenshot so the write tools can append the PNG and checklist to
their result, or a "Screenshot skipped" note when the preview tab is not
available. The preview page delays the PNG export by 600 ms right after
loading a new version so icon images finish loading.

* fix(mcp-server): review fixes for screenshot attached to create and edit results

* fix(mcp-server): Codex review fixes for the screenshot on write results

- A cleared canvas also gets the screenshot note when one was asked for
- The note for a tab that never polled says it may not have connected yet

* docs: list undo and custom drawing rules among the MCP features

* test(mcp-server): truncation check with named styles and compact cells after the merge

docs/ja/README_JA.md

@@ -117,6 +117,7 @@ claude mcp add drawio -- npx @next-ai-drawio/mcp-server@latest
 -   スクリーンショットツール。AI が描いた結果を確認して自分で直せる
 -   バージョン履歴、複数ページのダイアグラム、`.drawio`、`.png`、`.svg`、`.drawio.svg` でのダウンロード
 -   `~/.next-ai-drawio/` への自動保存。再起動後も続きから描ける
+-   AI に元に戻す操作を頼める。独自の作図ルールは `~/.next-ai-drawio/instructions.md` に書ける
 
 VS Code、Cursor などの設定は [MCP サーバーの README](../../packages/mcp-server/README.md) を参照してください。