Add Duo Chat Architecture Blueprint Documentation

Why is this change being made?

This merge request adds comprehensive documentation for GitLab's Duo Agentic Chat feature - the core conversational AI interface within the AI Panel. The documentation explains the current architecture challenges and defines the target architecture for data flow, streaming, and component design.

The key architectural decisions covered include: using Apollo Client's cache as the single source of truth for state management, decoupling WebSocket streaming from component lifecycle via a StreamManager service pattern, persisting agent and model selection in localStorage/cookies to support external triggers, and defining clear chat state terminology (New, Current, Old) for thread management.

The documentation also details how external triggers (like the troubleshoot button on pipeline pages) interact with the chat system, requiring certain state to be accessible before the chat component mounts. It covers the navigation rail integration where agent selection lives in the rail while model selection is in the input area.

The design addresses critical issues including streams being severed when users navigate away mid-response, inconsistent data flow patterns (props, events, Vuex, Vue.observable), and the duo-ui npm library slowing development. The blueprint documents the transition path to shadcn-based components mounted as web components for faster iteration.

Future enhancements documented include SharedWorker for even more resilient streaming that survives page navigation, and UI improvements like expandable input boxes for larger prompts.

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Commits

• Add Duo Chat Architecture Blueprint Documentation

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