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/modules/core/core-agents.adoc

@@ -0,0 +1,48 @@
+= core-agents
+
+== purpose
+
+This module has no source files. It exists as a buildable Gradle module with a `build.gradle` file but no Kotlin source code in the `src/main/kotlin` tree. It is a placeholder for future agent-related functionality.
+
+== responsibilities
+
+* None (no source files)
+
+== non-responsibilities
+
+* Everything (no source files)
+
+== key types
+
+No types exist in this module.
+
+== event flow
+
+This module does not interact with events.
+
+== integration points
+
+No dependencies (empty build.gradle with no `dependencies` block beyond the default plugins).
+
+== invariants
+
+n/a
+
+== PlantUML diagram
+
+[plantuml, core-agents, "png"]
+----
+include::../../diagrams/core-agents.puml[]
+----
+
+
+
+
+== known issues
+
+* The module compiles but produces no classes. If it is referenced as a dependency by another module, that module will fail to resolve imports.
+
+== open questions
+
+* What functionality is this module intended to contain? (Agent definitions, agent lifecycle, agent orchestration?)
+* Should it be removed or marked explicitly as pending?