Add Design Document for Modular Agent Architecture#22097

dewitt wants to merge 2 commits intogoogle-gemini:maingoogle-gemini/gemini-cli:mainfrom

dewitt:dewitt/issue-20995-1-agentic-refactor-docsdewitt/gemini-cli:dewitt/issue-20995-1-agentic-refactor-docsCopy head branch name to clipboard