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/mcp-server/src/preview/preview.js

@@ -300,11 +300,13 @@ async function poll() {
         // The tab's own push still on its way is not loaded back: the
         // canvas may have moved on since (an undo), and its answer follows
         const ownPush = pushesInFlight.includes(s.xml);
+        let justLoaded = false; // this poll put a new version on the canvas
         if ((forceReload || (s.version > currentVersion && !projectionExportActive && !ownPush)) && s.xml) {
             forceReload = false;
             projectionExportActive = false;
             currentVersion = s.version;
             loadDiagram(s.xml, true);
+            justLoaded = true;
         }
         // Handle sync request - server needs fresh state. After the load
         // above, so draw.io exports what it just loaded; never while a
@@ -354,6 +356,10 @@ async function poll() {
                 // Let draw.io render the loaded page before exporting
                 // (same proven settle delay as the AI-preview path).
                 setTimeout(fireExport, 600);
+            } else if (justLoaded) {
+                // A write with a screenshot: give the new diagram's external
+                // icon images a moment to load before the PNG is taken
+                setTimeout(fireExport, 600);
             } else {
                 fireExport();
             }