Assistant: define key usage metrics and add them to the key metrics report

Parent epic: #1288

As part of the beta launch, define the key metrics for assistant usage and add them to the key metrics report so it's reviewed alongside the other platform metrics.

As a starting point, consider the assistant funnel: **entry** (starting a session), **exit to the stepper** (handoff), and **launch in Galaxy** — and the conversions between them. Confirm which events are already captured and instrument any that aren't.

## Acceptance criteria

- An agreed set of assistant key metrics.
- The events needed for them are captured.
- Assistant usage appears in the key metrics report.