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/kernel/AGENTS.md

@@ -0,0 +1,30 @@
+# testing/kernel
+
+## Purpose
+
+Focused tests for `core:kernel` internals that require real event-store plumbing. Covers: approval-before-emit ordering race, retry coordinator behavior, replay inference provider, launch registration race conditions, workspace derivation from replay, and context feedback propagation.
+
+## Ownership
+
+Tests and `MockEventReplayer` helper in this submodule. Production modules exercised: `core:kernel`, `core:events`, `core:sessions`, `core:inference`, `core:approvals`, `core:context`, `core:transitions`, `core:validation`, `core:risk`, `core:tools`, `core:artifacts`, `core:artifacts-store`, `infrastructure:persistence`.
+
+## Local Contracts
+
+- `ApprovalRegisterBeforeEmitTest` pins the ordering invariant: approval listener must be registered before the event is emitted or the gate is missed.
+- `LaunchRegistrationRaceTest` pins that concurrent session launches do not corrupt registration state.
+- `koverVerify` is disabled for this submodule.
+
+## Work Guidance
+
+- Suspend tests use `runBlocking { ... }` or `kotlinx-coroutines-test`.
+- Race tests (`LaunchRegistrationRaceTest`) must not use `Thread.sleep()` — use coroutine barriers.
+
+## Verification
+
+```
+./gradlew :testing:kernel:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).