Expose shard count via cluster status API

## Request

When managing a sharded cluster across multiple environments, there is no easy programmatic way to determine how many shards are configured in a replica set. The shard count should be exposed through the cluster status API.

## Current state

`clusterStatus` exposes a `shard` field per-node (the shard index for that node), and internally `knownNodes.ts` builds a `server.shards` Map with all shard→node mappings. However, aggregate shard count is not surfaced in any API response.

## Proposed

Add `shardCount` (or `numberOfShards`) to the cluster/replication status response so consumers can easily determine topology without counting individual node `shard` fields themselves.

🤖 Filed by Claude on behalf of Kris.