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/replay/AGENTS.md

@@ -0,0 +1,30 @@
+# testing/replay
+
+## Purpose
+
+Pins replay determinism end-to-end: feeds a recorded event log through the full replay stack and asserts the resulting state is identical to what was produced during original execution. Covers: session replay, approval replay, validation replay, decision journal replay, refinement iteration replay, execution-plan-locked replay, repo-map replay, repo-knowledge replay, transition replay integration, and empty-session edge cases. Also exercises `DeterministicHarness` directly.
+
+## Ownership
+
+Tests in this submodule. Production modules exercised: `core:events`, `core:journal`, `core:kernel`, `core:context`, `core:sessions`, `core:transitions`, `core:validation`, `core:approvals`, `core:artifacts`, `infrastructure:persistence`, `infrastructure:workflow`.
+
+## Local Contracts
+
+- Hard invariant #8: replay must complete without any external service calls, live LLM, or network.
+- Hard invariant #9: environment observations (filesystem, clock, embedder) must be read from recorded events — replay tests must not re-query the environment.
+- Every replay test must compare final state byte-for-byte (or field-for-field) against a known-good fixture.
+
+## Work Guidance
+
+- Suspend tests use `runBlocking { ... }`.
+- Do not add `Thread.sleep()` or timing-sensitive assertions — replay is synchronous over a fixed event log.
+
+## Verification
+
+```
+./gradlew :testing:replay:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).