Add Python virtual environment setup instructions to CONTRIBUTING.md

[Copilot]

Step 2 in the Getting Started section said "Create a Python virtual environment" with no instructions or rationale.

Added:

• Why a venv matters (dependency isolation, reproducible builds)
• Instructions for using the Python Environments extension's "Create Environment" command (recommended approach)
• Alternative manual command-line instructions (python -m venv .venv) with activation commands for Linux/macOS, Windows CMD, and PowerShell
• Note about keeping it activated during development

[Copilot]

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

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/graphql
  • Triggering command: /usr/bin/gh gh issue list --state open --limit 5 --json number,title,labels (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)