feat(third_party): add Interactive Brokers plugin

[minupalaniappan]

Summary

• Add Interactive Brokers 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 (see Notes for the one deliberate deviation).
• Registers the plugin in .cursor-plugin/marketplace.json and the root README table.
• Stacked on feat(third_party): add S&P Global plugin #349 (minu/plugin-sp-global). Merge that first, or retarget this PR to main after it lands.

Endpoint	https://api.ibkr.com/v1/api/mcp-public
Auth	OAuth with dynamic client registration. Probed: 401 → api.ibkr.com, registration_endpoint present.
Grok visibility	all

MCP

{
  "mcpServers": {
    "interactive-brokers": {
      "type": "http",
      "url": "https://api.ibkr.com/v1/api/mcp-public"
    }
  }
}

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
2. webull feat(third_party): add Webull plugin #348
3. sp-global feat(third_party): add S&P Global plugin #349
   4. interactive-brokers feat(third_party): add Interactive Brokers plugin #350 ← this PR
4. meltwater feat(third_party): add Meltwater plugin #351
5. daloopa feat(third_party): add Daloopa plugin #352
6. excalidraw feat(third_party): add Excalidraw plugin #353
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 an account summary

Notes

• Endpoint deviates from Grok's catalog. Grok lists /mcp; IBKR documents /mcp-public for custom clients. Both share the same authorization server and behaved identically when probed.
• Trade instructions never become live orders; the user reviews them inside an IBKR platform.