Phase 7A+7C: Subsystem aggregator + Flow tracer (post-review fixes)

test/fixtures/system-docs/architecture.md

@@ -0,0 +1,17 @@
+# Mock Architecture Documentation
+
+## Purpose
+This is a test fixture simulating a real repo's architecture.md file.
+
+## Layered Architecture
+The system follows a layered architecture:
+- **Channels** receive inbound messages from external platforms
+- **Gateway** manages sessions and routes requests
+- **Agents** execute AI model interactions
+- **Config** provides centralized configuration
+- **Utils** provides shared infrastructure (logging, crypto, filesystem)
+
+## Design Decisions
+- Gateway and Agents have a deliberate circular dependency for session refresh workflows
+- Config is loaded lazily and cached in memory
+- Utils are stateless pure functions with no domain logic