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/app-tui.puml

@@ -0,0 +1,19 @@
+@startuml
+[KeyResolver] -> [RootReducer] : Action
+[WsClient.messages] -> [RootReducer] : ServerEventReceived
+[WsClient.connection] -> [RootReducer] : Connected/Disconnected/RetryScheduled
+
+[RootReducer] --> [SnapshotPhaseReducer]
+[RootReducer] --> [InputReducer]
+[RootReducer] --> [SessionsReducer]
+[RootReducer] --> [ConnectionReducer]
+[RootReducer] --> [ProviderReducer]
+
+[RootReducer] -> [EffectDispatcher] : List<Effect>
+[EffectDispatcher] -> [WsClient] : SendWs
+[EffectDispatcher] -> [System] : Quit
+
+clock over [WsClient] : exponential backoff reconnect
+
+[RootReducer] --> [Renderer] : TuiState
+@enduml