feat(third_party): add Excalidraw plugin

[minupalaniappan]

Summary

• Add Excalidraw as a third_party/ marketplace plugin, in the Attio (Add Attio third-party MCP plugin #339) / Hunter (Add Hunter third-party MCP plugin (API key header auth) #340) shape.
• One of nine hosted-MCP connectors that grok.com offers publicly (GrowthBook visibility all) and the Cursor marketplace does not. The server URL matches Grok's production connector_catalog.mcp_server_url.
• Registers the plugin in .cursor-plugin/marketplace.json and the root README table.
• Based on main; bottom of the stack now that feat(third_party): add Gamma plugin #347, feat(third_party): add Webull plugin #348, feat(third_party): add S&P Global plugin #349, feat(third_party): add Interactive Brokers plugin #350, feat(third_party): add Meltwater plugin #351, feat(third_party): add Daloopa plugin #352 have merged.

Endpoint	https://mcp.excalidraw.com/mcp
Auth	None. Probed: initialize and tools/list succeed unauthenticated (tools: create_view, export_to_excalidraw, save_checkpoint, read_checkpoint, read_me).
Grok visibility	all

MCP

{
  "mcpServers": {
    "excalidraw": {
      "type": "http",
      "url": "https://mcp.excalidraw.com/mcp"
    }
  }
}

Stack

Split from #345 into one PR per plugin. Independent plugins come first; the two with external prerequisites (Coinbase, BigQuery) are last so they never block the others.

1. gamma feat(third_party): add Gamma plugin #347 ✅ merged
2. webull feat(third_party): add Webull plugin #348 ✅ merged
3. sp-global feat(third_party): add S&P Global plugin #349 ✅ merged
4. interactive-brokers feat(third_party): add Interactive Brokers plugin #350 ✅ merged
5. meltwater feat(third_party): add Meltwater plugin #351 ✅ merged
6. daloopa feat(third_party): add Daloopa plugin #352 ✅ merged
   7. excalidraw feat(third_party): add Excalidraw plugin #353 ← this PR
7. coinbase feat(third_party): add Coinbase plugin #354
8. google-cloud-bigquery feat(third_party): add Google Cloud BigQuery plugin #355

Test plan

• node scripts/validate-plugins.mjs passes on this branch
• Endpoint probed for OAuth discoverability and dynamic client registration; the auth shape above is what the server actually supports
• README has Install / MCP / Before you connect / What agents can do / Notes / Docs / License, with capabilities sourced from vendor docs
• marketplace.json, plugin.json, and README table descriptions are identical
• Logo visually reviewed at 192×192
• Install from Cursor Settings → Plugins, complete sign-in, and smoke test create_view

Notes

• This is the open-source Excalidraw MCP server, not the Excalidraw+ workspace API (api.excalidraw.com/api/v1/mcp).
• The bare origin https://mcp.excalidraw.com 308-redirects to /mcp; the plugin uses the final URL.

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Note

Low Risk
Additive marketplace entry and documentation only; no changes to core app logic, with traffic going to a public third-party MCP endpoint.

Overview
Adds a new Excalidraw third-party marketplace plugin so Cursor agents can draw and export hand-drawn diagrams from chat via Excalidraw’s hosted MCP server.

The plugin lives under third_party/excalidraw/ with the usual manifest (plugin.json), mcp.json pointing at https://mcp.excalidraw.com/mcp (HTTP, no auth), README/changelog/license, and Cursor 3.13.0 minimum. Excalidraw is registered in .cursor-plugin/marketplace.json and the root README plugin table with the same description as other integrations.

Docs call out the open-source MCP endpoint (not Excalidraw+’s authenticated API) and agent capabilities: create diagram views, export .excalidraw files, and session checkpoints.

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