Docs: Workaround for minimal terminal for Duo Agentic Chat in IDEs

What does this MR do?

This change adds troubleshooting guidance to help users fix issues where GitLab's AI agents get stuck in endless loops when trying to run terminal commands. The problem occurs when fancy shell themes and customizations interfere with the AI's ability to properly execute commands and read their results. The documentation now provides step-by-step instructions for users to configure their terminal shells (ZSH and Bash) to automatically detect when they're being used by GitLab's AI agents in code editors like VS Code or JetBrains, and switch to a simpler, minimal setup that won't cause conflicts. This allows the AI agents to work properly while still preserving the user's preferred terminal experience for regular use.

Related issues

gitlab-vscode-extension#2070 (closed)

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