google-calendar MCP server turned down — productivity plugin broken

## Problem

The `google-calendar` MCP connection used by the `productivity` plugin stopped working today (2026-04-17). When Claude Code tries to connect, the MCP server returns a \"Server Turned Down\" page.

## Error message

```
Server Turned Down
This MCP server has been turned down.

Please use https://calendarmcp.googleapis.com/mcp/v1 instead.
```

The error page was served from `gcal.mcp.claude.com` (the install URL used by the plugin).

## Root cause

The original MCP server at `gcal.mcp.claude.com` has been shut down. The suggested replacement endpoint (`https://calendarmcp.googleapis.com/mcp/v1`) does not support **dynamic client registration**, which Claude Code requires for OAuth-based MCP servers. This means there is currently no path to reconnect to Google Calendar through this plugin.

## Impact

The `productivity` plugin's Google Calendar integration is completely broken. Any skills or briefings that depend on calendar data (e.g., daily briefings, meeting context) will fail silently or error.

## Steps to reproduce

1. Install the `productivity` plugin in Claude Code
2. Attempt to connect the `google-calendar` MCP server
3. See \"Failed to reconnect to google-calendar\" error and the \"Server Turned Down\" page

## Expected behavior

The plugin should either:
- Point to an updated MCP endpoint that supports dynamic client registration, or
- Surface a clear migration guide explaining how to reconnect using the new `https://calendarmcp.googleapis.com/mcp/v1` endpoint (including how to handle the auth flow without dynamic client registration)

## Environment

- Claude Code version: current (as of 2026-04-17)
- Platform: macOS