Refactor load testing scripts to support Bash and PowerShell

- Updated README.md to reflect changes in load testing commands and added usage examples for Bash scripts.
- Converted PowerShell load testing scripts to Bash equivalents for better cross-platform compatibility.
- Added new Bash scripts: run-loadtests.sh, summarize-artillery.sh, and summarize-k6.sh to mirror existing PowerShell functionality.
- Updated admin-api.js and ws-chat.yml for consistency and improved performance metrics reporting.
- Enhanced error handling and output formatting in summary scripts for better readability.

scripts/cache-hit-ratio.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+# cache-hit-ratio.sh — Thin wrapper that runs the Node.js cache-hit-ratio tool.
+# Mirrors scripts/cache-hit-ratio.ps1.
+set -euo pipefail
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+exec node "$script_dir/cache-hit-ratio.js" "$@"