[telemetry] Agent/MCP telemetry: cooked tables, data quality, and dashboard cleanup

## Problem

- There are no cooked tables for agent and MCP usage, even though agent usage is one of the fastest-growing parts of our traffic.
- The MCP tools section of the current Power BI dashboard looks wrong: a single data point, and only GPT-5.3 showing, when users are likely on different models.
- There are stale agent/MCP tables in the current dashboard that no longer reflect how we want to report.

## What we'll do

- Add cooked tables for agent and MCP usage.
- Investigate and fix the MCP data quality issue so the section reflects real usage and real model distribution.
- **Clean up the existing agent/MCP tables in the current Power BI dashboard.**

## Done when

- Agent and MCP have proper cooked tables.
- The MCP section reflects real usage.
- Stale agent/MCP tables are removed from the current dashboard.