docs: add module documentation in AsciiDoc with PlantUML diagrams

Generated by per-group subagents covering all 52 modules across:
core (18), infrastructure (10), apps (5), interfaces (3),
plugins (7), testing (9)

Documentation format:
- AsciiDoc (.adoc) files in docs/modules/<group>/
- PlantUML (.puml) files in docs/diagrams/
- .adoc files reference diagrams via include:: directives

Each doc covers: purpose, responsibilities, non-responsibilities,
key types, event flow, integration points, invariants, PlantUML
diagram, known issues, and open questions.

docs/diagrams/test-integration.puml

@@ -0,0 +1,13 @@
+@startuml
+[SessionOrchestratorIntegrationTest] -> [DefaultSessionOrchestrator] : run(sessionId, graph, config)
+[DefaultSessionOrchestrator] --> [EventStore] : append events
+[DefaultSessionOrchestrator] --> [MockEngines] : resolve/infer/validate/approve
+[Test] <- [EventStore] : read events
+[Test] <- [SessionRepository] : get session state
+
+[ValidationPipelineIntegrationTest] -> [ValidationPipeline] : validate(payload)
+[ValidationPipeline] -> [MockValidator1] : validate
+[ValidationPipeline] -> [MockValidator2] : validate
+[MockValidator*] --> [ValidationPipeline] : ValidationSection
+[Test] <- [ValidationPipeline] : ValidationReport
+@enduml