Add external plugin markdstage (#4176)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

.github/plugin/marketplace.json

@@ -1274,6 +1274,31 @@
       "description": "Complete toolkit for building Model Context Protocol (MCP) servers in Kotlin using the official io.modelcontextprotocol:kotlin-sdk library. Includes instructions for best practices, a prompt for generating servers, and an expert chat mode for guidance.",
       "version": "1.0.0"
     },
+    {
+      "name": "markdstage",
+      "description": "Create, edit, present, and export Markdown slides in a GitHub Copilot Canvas.",
+      "version": "4.2.4",
+      "author": {
+        "name": "Kazuki Ota",
+        "url": "https://github.com/runceel"
+      },
+      "repository": "https://github.com/runceel/MarkdStage",
+      "license": "MIT",
+      "keywords": [
+        "canvas",
+        "copilot-canvas",
+        "markdown",
+        "slides",
+        "presentation"
+      ],
+      "source": {
+        "source": "github",
+        "repo": "runceel/MarkdStage",
+        "path": ".github/plugin/markdstage",
+        "ref": "v4.2.4",
+        "sha": "34621f7217fbf2a10499c229229df4c86241095d"
+      }
+    },
     {
       "name": "mcp-m365-copilot",
       "source": "plugins/mcp-m365-copilot",