Implement command for explaining terminal output (AI:GW + Monolith)

Problem to Solve

Currently, Duo Chat is not available to users in the terminal. We want to pave a path forward for the user to select output in the terminal and ask chat to explain what the output is.

• Required by gitlab-vscode-extension#1423 (closed)

Proposed Solution

With considerations of using Injected Context as referenced here: gitlab-vscode-extension#1423 (comment 2189362524) -- implementing the command independent of providing it as a context, for now.

1. Add the command to the backend /explain_terminal
2. Append the terminal output to the prompt itself.