feat(plugins): add Google Chat MCP plugin

[n2parko]

Why

Package Google's official Google Chat MCP connector for Cursor and Grok Bot in response to in-product asks. The sample request came from #grok-bot-extensibility (channel C0BQPM51RV4, message 1789956769493339).

Approved for packaging by Kevin Niparko (Product) in a Slack GDM on 2026-09-21; the digest row was a packaging candidate in #grok-bot-extensibility.

What changed

• Added the google-chat third-party plugin and marketplace entry.
• Configured the HTTP MCP endpoint at https://chatmcp.googleapis.com/mcp/v1.
• Uses Google OAuth 2.0; no API-key headers are configured.
• Documented that Google Chat MCP is currently a Google Workspace Developer Preview.
• Added the official Google Chat product icon with source attribution.

Test plan

• Validate the plugin and marketplace manifests against repository schemas (node scripts/validate-plugins.mjs).
• Install the Google Chat plugin from the marketplace.
• Complete the Google OAuth flow.
• Confirm the Google Chat MCP tools are listed.
• If the connected Workspace permits it, send a test Chat action.