Add random ports guidance for detached mode

[Copilot]

Adds best practices for using random port assignment with detached mode, as requested in review feedback.

Based on the v2.2.0 source in dotnet/dev-proxy (ProxyConfiguration.cs, DevProxyCommand.cs, detached PR #1533, port-logging fix PR #1576):

• --port 0 / --api-port 0 — OS-assigned ephemeral ports to avoid conflicts when running parallel instances
• --as-system-proxy false — isolate each instance so it only intercepts explicitly configured traffic
• devproxy status — discover actual assigned port, API URL, PID for a running detached instance

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

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• learn.microsoft.com
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