Add new task queue kind to send worker commands (#763)

## What

Add `TASK_QUEUE_KIND_WORKER_COMMANDS` (value 3) to `TaskQueueKind` enum.

## Why

Worker-commands task queues are used for server-to-worker communication
(e.g. activity cancellations). They need to be distinguished from
user-defined queues so the server can handle them differently in metrics
and other areas. SDKs will set this kind when polling worker-commands
queues.

## How did you test it?

Proto-only change, no runtime behavior yet.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: rkannan82