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/transitions/AGENTS.md

@@ -0,0 +1,29 @@
+# testing/transitions
+
+## Purpose
+
+Verifies `core:transitions` — the FSM/workflow graph layer. Covers: `DefaultTransitionResolver` correctness, transition event serialization round-trips, and fuzz coverage over random graph topologies.
+
+## Ownership
+
+Tests in this submodule. Production modules exercised: `core:transitions`, `core:events`, `core:inference`.
+
+## Local Contracts
+
+- `TransitionEventSerializationTest` pins that every transition event survives a serialize → deserialize round-trip without data loss; must be updated when new transition event types are added.
+- `TransitionFuzzTest` generates random graphs; add new resolver logic → extend the fuzz domain.
+
+## Work Guidance
+
+- Suspend tests use `runBlocking { ... }`.
+- This is a narrow module — keep tests focused on graph resolution and serialization; cross-cutting concerns belong in `testing:integration` or `testing:deterministic`.
+
+## Verification
+
+```
+./gradlew :testing:transitions:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).