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

packages/claude-plugin/README.md

@@ -123,6 +123,7 @@ Claude Code <--stdio--> MCP Server <--http--> Browser (draw.io)
 | `DRAWIO_LANG` | unset | Language of the draw.io editor, such as `en`, `zh`, `zh-tw`, `ja` or `de`. Unset, draw.io chooses (the browser language on `embed.diagrams.net`, English on a self-hosted draw.io) and the user can change it under **Extras > Language** |
 | `DRAWIO_UI` | unset | draw.io theme: `kennedy`, `atlas`, `dark`, `min`, `sketch` or `simple`. Unset, the user picks one under **Extras > Theme** |
 | `DRAWIO_DARK` | `auto` | Dark mode of the draw.io editor: `auto` follows the system, `1` forces dark, `0` forces light |
+| `DRAWIO_AUTO_SCREENSHOT` | unset | Set to `true` to attach a screenshot to every `create_new_diagram` and `edit_diagram` result (costs 2 to 10 s per call) |
 | `DEBUG` | unset | Set to `true` to log debug messages |
 
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