Add Attio third-party MCP plugin

[SamSokolin]

Summary

• Add a marketplace plugin under third_party/attio/, pointing at Attio's official hosted MCP server (https://mcp.attio.com/mcp) over Streamable HTTP. Source of truth: Attio MCP overview and the Attio MCP help center.
• Auth is OAuth with Attio user login (no API keys, client IDs, or secrets). mcp.json is URL-only, matching Hunter (Add Hunter third-party MCP plugin #317), Clay, Intercom, and Calendly.
• Confirmed Attio is not already packaged in this repo (no attio path or marketplace entry) and is not listed on cursor.com/marketplace.
• Register the plugin in .cursor-plugin/marketplace.json and the root README table.
• Logo is Attio's official mark from the attio GitHub organization (192×192 PNG).

MCP

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

Read operations are auto-approved; writes request confirmation. Optional read-only SQL (query-particle-sql) is plan-dependent.

Test plan

• Plugin schema validation passes (node scripts/validate-plugins.mjs)
• Install from Cursor Settings → Plugins or /add-plugin attio
• Complete Attio OAuth sign-in
• Smoke test a read (e.g. list open deals or workspace members), then a confirmed write (e.g. create a task)

Notes

• This is packaging only — no local stdio wrapper. Attio ships the hosted server.
• Marketplace listing after merge still needs the usual Cursor marketplace publish step (same as other third_party/* connectors).
• Logo asset: official GitHub org avatar resized to 192×192. If Attio brand prefers the light/wordmark variant from attio.com/brand, that can be swapped in a follow-up.