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/projections/AGENTS.md

@@ -0,0 +1,30 @@
+# testing/projections
+
+## Purpose
+
+Verifies that reducers and projectors rebuild state correctly from event sequences. Covers: `DefaultSessionReducer`, `OrchestrationReducer`/`Projector`, `InferenceReducer`/`Projector`, `ArtifactReducer`, `ContextReducer`/`Projector`, `RouterProjector`, `DecisionJournalProjector`, `SessionProjector`, `ProjectMemoryDistiller`, and `PreemptSteeringJournal`.
+
+## Ownership
+
+Tests in this submodule. Production modules exercised: `core:events`, `core:sessions`, `core:transitions`, `core:context`, `core:inference`, `core:kernel`, `core:artifacts`, `core:router`, `core:journal`.
+
+## Local Contracts
+
+- Projections must be rebuilt from events only — no test may assert on a persisted State object.
+- Reducers are pure functions: same events → same state, always. Tests must verify this property explicitly (run twice, compare).
+- Any new reducer or projector added to a `core:*` module needs a corresponding test here.
+
+## Work Guidance
+
+- Suspend tests use `runBlocking { ... }`.
+- Keep event sequences in tests minimal — use `testing:fixtures` builders rather than raw event construction.
+
+## Verification
+
+```
+./gradlew :testing:projections:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).