feat(third_party): add Square plugin

[djiang-jq]

Adds a square plugin that connects to Square's official remote MCP server, hosted by Block.

What's included

• third_party/square/mcp.json: HTTP server at https://mcp.squareup.com/mcp
• plugin.json, README, CHANGELOG, LICENSE, and logo (Square's official mark, from the square GitHub organization)
• Marketplace and root README entries

Auth

OAuth with dynamic client registration and PKCE. Confirmed from the server: an unauthenticated initialize returns 401 with a resource_metadata pointer, and /.well-known/oauth-authorization-server advertises a registration_endpoint and S256. There is no token or client ID to configure.

Notes for reviewers

• The remote server reaches production data only and supports writes when *_WRITE scopes are approved. The README documents both.
• Square keeps an allowlist of MCP clients for OAuth registration. If Cursor isn't on it, sign-in may be rejected until Square adds it. I did not test an end-to-end sign-in.
• Square labels the server as beta.

Validation

• node scripts/validate-plugins.mjs passes

Docs: https://developer.squareup.com/docs/mcp

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Note

Low Risk
Repo changes are manifest and docs only; operational risk is users granting write scopes on live Square seller data via the remote MCP server.

Overview
Adds a new square Cursor marketplace plugin under third_party/square/ that wires agents to Square’s hosted MCP endpoint at https://mcp.squareup.com/mcp via mcp.json and plugin.json (integrations category, Cursor 3.13.0+).

Registers the plugin in .cursor-plugin/marketplace.json and the root README integrations table, with standard packaging (README, CHANGELOG, MIT LICENSE, logo). Auth is documented as OAuth 2.1 (dynamic client registration + PKCE) with no manual token setup; the README calls out production-only access, optional write scopes, beta status, and Square’s MCP client allowlist.

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