docs(dox): build out the DOX AGENTS.md hierarchy

Root Child DOX Index assembled, plus a per-module AGENTS.md across the tree
(core/*, infrastructure/*, apps/*, testing/*, and docs/examples/frontend/etc),
each following the DOX section shape: Purpose, Ownership, Local Contracts,
Work Guidance, Verification, Child DOX Index.

core/artifacts/AGENTS.md

@@ -0,0 +1,37 @@
+# core/artifacts — AGENTS.md
+
+## Purpose
+
+Structured output model for workflow artifacts: defines artifact kinds, tracks lineage and relationships between artifacts, and provides the projector/reducer/repository stack for artifact state.
+
+## Ownership
+
+CORREX kernel team.
+
+## Local Contracts
+
+- `Artifact` — core domain type, carries `ArtifactKind`, payload, and metadata.
+- `ArtifactKind` / `ArtifactKindRegistry` — extensible registry; built-in kinds: `ConfigArtifactKind`, `FileWrittenKind`, `ProcessResultKind`.
+- `ArtifactLineage` / `ArtifactRelationship` / `ArtifactRelationshipType` — DAG of artifact provenance.
+- `ArtifactState` rebuilt from events via `DefaultArtifactReducer` (in `core:events` `ArtifactEvents.kt`) + `ArtifactProjector`.
+- `TypedArtifactSlot<T>` — typed accessor for well-known artifact slots.
+- `ArtifactSerializationModule` — registers artifact polymorphic types; must be included in serialization setup.
+- Hard Invariant #1: artifact state is always rebuilt from events. `ArtifactRepository` wraps `EventReplayer<ArtifactState>`.
+
+## Work Guidance
+
+- Follow the standard Events→State→Reducer→Projector→Repository pattern (see `core/AGENTS.md`).
+- New artifact kinds must be registered in `ArtifactKindRegistry` and their payloads in `ArtifactSerializationModule`.
+- `JsonSchema` is a local utility for schema representation; do not reach into external schema libraries.
+
+## Verification
+
+```bash
+./gradlew :core:artifacts:test --rerun-tasks
+```
+
+Reducer tests in `testing/projections/ArtifactReducerTest.kt`.
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).