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.
• Based on main; bottom of the stack now that feat(third_party): add Gamma plugin #347 (Gamma), feat(third_party): add Webull plugin #348 (Webull), and feat(third_party): add S&P Global plugin #349 (S&P Global) have merged.

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 ✅ 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 ← 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.

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Note

Medium Risk
Introduces a brokerage OAuth MCP integration (positions, balances, draft trades); risk is mainly user consent and agent use of financial tools, though this PR only adds manifests and docs, not Cursor runtime logic.

Overview
Adds a new interactive-brokers third-party Cursor plugin so agents can use Interactive Brokers’s hosted MCP server for portfolio and trading workflows.

The change registers the plugin in .cursor-plugin/marketplace.json and the root README table, and adds third_party/interactive-brokers/ with manifest, mcp.json (HTTP → https://api.ibkr.com/v1/api/mcp-public), install/docs README, changelog, and MIT license. Auth is documented as OAuth at connect time (no API keys in the plugin). Capabilities cover accounts, activity, market data, and draft trade instructions reviewed in IBKR—not auto-submitted orders.

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