greenhouse: add plugin for Greenhouse's official remote MCP server

[ericgyl817]

Customer impact: PGT-4591: the talent team asked for Greenhouse in their shared bot, and users were hand-adding the MCP URL; this adds the official Greenhouse plugin to the catalog, with the Site Admin setup and Cursor redirect URLs documented so sign-in does not end on Greenhouse's "Unable to authorize" page.

• Adds third_party/greenhouse pointing at Greenhouse's official remote MCP server https://mcp.us.greenhouse.io/mcp (OAuth PKCE + dynamic client registration, scope greenhouse:mcp_client), plus its marketplace entry (PGT-4591).
• Why: Greenhouse is the enterprise ATS default and users already hand-add this URL; the README covers the org prerequisite (Site Admin enables scopes in Dev Center → MCP Access) and Cursor's redirect URLs for the Clients allowlist.
• Regression: no, additive plugin; validate-plugins.mjs passes and the backend's parsePluginMcpConfig parses the config.

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Note

Low Risk
Additive marketplace plugin and MCP config only; no changes to core auth or application logic beyond documenting OAuth prerequisites.

Overview
Adds a Greenhouse Cursor plugin so agents can use Greenhouse Recruiting via Greenhouse’s official remote MCP server (https://mcp.us.greenhouse.io/mcp), with OAuth (PKCE, dynamic client registration, greenhouse:mcp_client scope).

The change is additive: a new third_party/greenhouse package (plugin.json, mcp.json, README, changelog, license) and a matching entry in the plugin marketplace. The README documents Site Admin MCP Access setup and Cursor redirect URLs for OAuth allowlisting.

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