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-store/AGENTS.md

@@ -0,0 +1,29 @@
+# core/artifacts-store — AGENTS.md
+
+## Purpose
+
+Thin persistence interface (`ArtifactStore`) for artifact blobs. Decouples the artifact domain model from the storage adapter so `infrastructure/` can supply the real implementation.
+
+## Ownership
+
+CORREX kernel team.
+
+## Local Contracts
+
+- `ArtifactStore` — single interface; implementations live in `infrastructure/`.
+- No domain logic here. No event sourcing stack — this is a raw store abstraction, not a projection.
+
+## Work Guidance
+
+- Keep this module minimal. Any new method on `ArtifactStore` must have a corresponding implementation in every adapter under `infrastructure/`.
+- Do not add domain types here; those belong in `core:artifacts`.
+
+## Verification
+
+```bash
+./gradlew :core:artifacts-store:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).