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/core-config.puml

@@ -0,0 +1,18 @@
+@startuml
+!theme plain
+package "core-config" {
+  object ConfigLoader
+  class CorrexConfig
+  class ServerConfig
+  class TuiConfig
+  class CliConfig
+  class ApprovalConfig
+  class ToolsConfig
+}
+ConfigLoader --> CorrexConfig : loads
+CorrexConfig *--> ServerConfig
+CorrexConfig *--> TuiConfig
+CorrexConfig *--> CliConfig
+CorrexConfig *--> ApprovalConfig
+CorrexConfig *--> ToolsConfig
+@enduml