feat(third_party): add Gamma plugin

[minupalaniappan]

Summary

• Add Gamma 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.

Endpoint	https://mcp.gamma.app/mcp
Auth	OAuth with dynamic client registration. Probed: 401 → authorization server auth.gamma.app, registration_endpoint present.
Grok visibility	all

MCP

{
  "mcpServers": {
    "gamma": {
      "type": "http",
      "url": "https://mcp.gamma.app/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 #347 ← this PR
2. webull #348
3. sp-global #349
4. interactive-brokers #350
5. meltwater #351
6. daloopa #352
7. excalidraw #353
8. coinbase #354
9. google-cloud-bigquery #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 get_themes

Notes

• Auth is OAuth 2.0 with DCR; no client ID or API key to configure.
• Generation is asynchronous; tools return an ID the agent polls.