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.

testing/fixtures/AGENTS.md

@@ -0,0 +1,30 @@
+# testing/fixtures
+
+## Purpose
+
+Shared test data library. Not a test runner — contains no test classes of its own (except `MockToolsTest` which validates the mocks themselves). Provides: mock implementations (`MockInferenceProvider`, `MockTokenizer`, `MockEventReplayer`, `MockSessionRepository`), stub tools (`EchoTool`, `FailingTool`, `RejectedTool`), fixture builders (`EventFixtures`, `InferenceFixtures`, `WorkflowFixtures`, `ApprovalFixtures`, `CycleFixtures`, `ContextFixtures`, `TransitionFixtures`), and `DeterministicHarness`.
+
+## Ownership
+
+This submodule is a library dependency for other `testing/*` submodules. Production modules it wraps: `core:events`, `core:transitions`, `core:approvals`, `core:sessions`, `core:tools`, `core:inference`, `core:context`, `core:kernel`, `core:validation`, `infrastructure:persistence`.
+
+## Local Contracts
+
+- All mocks must remain stateless or explicitly reset between uses — callers cannot assume shared state across tests.
+- `DeterministicHarness` must not make live LLM or network calls.
+- `koverVerify` is disabled (library code, not independently measurable coverage).
+
+## Work Guidance
+
+- Add fixtures here only when two or more `testing/*` submodules need the same setup. Single-use helpers stay local to the submodule that needs them.
+- No domain logic in fixtures — only wiring and canned data.
+
+## Verification
+
+```
+./gradlew :testing:fixtures:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).