feat(third_party): add Coinbase plugin

[minupalaniappan]

Summary

• Add Coinbase 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.
• Stacked on feat(third_party): add Excalidraw plugin #353 (minu/plugin-excalidraw). Merge that first, or retarget this PR to main after it lands.

Endpoint	https://agents.coinbase.com/mcp
Auth	OAuth via CLIENT_ID / CLIENT_SECRET plugin variables. Probed: 401 → login.coinbase.com, which publishes no registration_endpoint, and Cursor has no pre-registered Coinbase client.
Grok visibility	all

MCP

{
  "mcpServers": {
    "coinbase": {
      "url": "https://agents.coinbase.com/mcp",
      "auth": {
        "CLIENT_ID": "${CLIENT_ID}",
        "CLIENT_SECRET": "${CLIENT_SECRET}"
      }
    }
  }
}

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
5. meltwater feat(third_party): add Meltwater plugin #351
6. daloopa feat(third_party): add Daloopa plugin #352
7. excalidraw feat(third_party): add Excalidraw plugin #353
   8. coinbase feat(third_party): add Coinbase plugin #354 ← this PR
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 / Setup / 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 listing portfolios after entering client credentials

Notes

• Follows the Gong / HubSpot / DocuSign pattern: the user registers an OAuth client in the Coinbase Developer Platform portal and enters its credentials in Dashboard → Plugins → Configure. The README lists both redirect URIs.
• Orders placed through this server are live; Coinbase recommends scoping the agent to a dedicated portfolio.