Chat: include selected subagent kind in runSubagent telemetry

## Summary

Add a privacy-safe selected/resolved subagent kind to production telemetry for `runSubagent` executions so behavior changes such as `generalPurpose` -> `Explore` can be confirmed and impact-sized online.

## Request

For each `runSubagent` execution, emit a queryable, privacy-reviewed categorical field representing the resolved subagent kind.

The field should:

- distinguish built-in agents such as `generalPurpose` and `Explore`;
- use a stable identifier rather than display text;
- classify arbitrary user/custom agents as `custom` (or another coarse value), rather than emitting their names;
- use `unknown` when resolution is unavailable;
- remain joinable with existing model, release-channel, VS Code version, request, token, and latency dimensions.

If the runtime/SDK owns the relevant tool-call telemetry schema, please route or coordinate the schema change there while keeping the VS Code analysis field queryable.

## Related

- #325259
- #326984