docs(architecture): baseline current design (#109)

Capture the brownfield architecture and its relationship to the project vision.\n\n- add the current-state SAD and five retrospective ADRs\n- classify vision vectors against implementation evidence\n- record requirements, risks, gates, and construction handoff

.aiwg/requirements/UC-002.md

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+# UC-002 — Connect a Model or Coding Agent
+
+**Primary actor:** Operator or integrator  
+**Goal:** Use a hosted provider, local OpenAI-compatible model, or connected coding-agent CLI as the reasoning backbone.
+
+## Main Flow
+
+1. User selects/configures a provider or local agent.
+2. System resolves provider/model capabilities and credentials.
+3. Requests are routed with timeout and fallback behavior.
+4. Tool-capable orchestration consumes responses and records outcomes.
+
+**Implementation coverage:** `src/llm/`, `src/config/`, `src/agent/`, provider-model registry, setup and fallback tests.