docs: add proxy mode env vars to ENVIRONMENT_VARIABLES.md and link gateway compatibility reference from README#2541

lpcox merged 2 commits into

maingithub/gh-aw-mcpg:mainfrom

copilot/fix-missing-proxy-mode-variablesgithub/gh-aw-mcpg:copilot/fix-missing-proxy-mode-variablesCopy head branch name to clipboard