[telemetry] Retain raw events into cooked tables and share Kusto queries as team notes

## Problem

Raw events are only retained for 90 days. Any question with a long horizon — year-over-year growth, cohort retention, whether a feature is being adopted over time — cannot be answered at all.

The workaround so far has been to cook a new table for every chart, which does not scale and loses data whenever we get the shape wrong.

## What we'll do

- Retain raw events into cooked table(s) so we keep long history, preserving enough of the event properties to support ad hoc analysis rather than a fixed set of columns.
- Store reusable Kusto queries as shared, versioned notes in the `azure-dev-tools` repo, so the team can self-serve answers instead of requesting a new table each time.

## Done when

- A retained table exists with history beyond 90 days.
- There is a query/notes library in `azure-dev-tools` that the team actually uses.
- A new question can be answered without adding a new cooked table.