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/deterministic/AGENTS.md

@@ -0,0 +1,30 @@
+# testing/deterministic
+
+## Purpose
+
+Pins determinism invariants across the core. Every test here asserts that a given computation produces identical output for identical event input — no randomness, no external I/O. Covers: router reducer and projector, session replay, context compression, prompt renderer ordering, token budget enforcement, approval engine, validation pipeline short-circuit, context pack builder, and L3 metadata rehydration.
+
+## Ownership
+
+Tests in this submodule. Production modules exercised: `core:events`, `core:router`, `core:sessions`, `core:transitions`, `core:validation`, `core:approvals`, `core:context`, `core:inference`, `infrastructure:persistence`.
+
+## Local Contracts
+
+- No test in this submodule may call an external service or live LLM — hard invariant #8 (replay is environment-independent).
+- Every tested path must produce byte-identical state for the same event log on repeated runs.
+- `PromptRendererOrderingTest` pins the rule that all SYSTEM-role entries fold into the leading system message; non-system messages order by `ContextEntry.ordinal`.
+
+## Work Guidance
+
+- Suspend tests use `runBlocking { ... }`.
+- If a new deterministic component is added to `core:router` or `core:context`, add a matching test here.
+
+## Verification
+
+```
+./gradlew :testing:deterministic:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).