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/contracts/AGENTS.md

@@ -0,0 +1,31 @@
+# testing/contracts
+
+## Purpose
+
+Cross-module interface contracts and shared contract test fixtures. Two responsibilities:
+1. **Contract tests** — verify that model types (`StageId`, `TransitionId`, `WorkflowGraph`, `ContextSnapshot`, `ValidationReport`) and provider implementations (`InferenceProvider`, inference router, resource probes) satisfy their declared interfaces.
+2. **Reusable fixtures** — `testFixtures` source set exports `ProjectionContractTest`, `ReplayContractTest`, `SessionReplayContractTest`, `CountingProjectionContractTest`, and `NoopArtifactStore` for use by other test submodules.
+
+## Ownership
+
+Tests and test fixtures here. Production modules exercised: `core:events`, `core:transitions`, `core:sessions`, `core:validation`, `core:approvals`, `core:artifacts`, `core:kernel`, `core:inference`, `core:context`, `infrastructure:inference:commons`.
+
+## Local Contracts
+
+- Contract test fixtures in `testFixtures/` are the canonical base for projection and replay contract verification — other submodules inherit from them.
+- `koverVerify` is disabled for this submodule (fixture code is not independently measurable).
+
+## Work Guidance
+
+- Suspend tests use `runBlocking { ... }`.
+- `ConcurrencyRunner` and `DeterministicBarrier` in `main/` are utilities for concurrency contracts — do not add state to them.
+
+## Verification
+
+```
+./gradlew :testing:contracts:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).