mixpanel: AI-optimize MCP action set

Mixpanel today can only send events in. An agent can write data but can't read any of the analytics back out. We'd like this improved and new tools added so an agent can actually answer product questions, and so each tool is easy for an agent to reach for and get right.

A few things an agent can't do today, for example:
- Run a query or pull a report — e.g. "how many signups this week?"
- Look up a user profile and their activity
- Get funnel or retention numbers
- List the events or properties being tracked

**API reference:** https://docs.mixpanel.com/reference/overview

**Add marker:**
When a component is AI-optimized, add this as the first line of the file:
```
// x-pd-ai: optimized
```
Add it to each action file and to the app's `.mjs` file.