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.

interfaces/AGENTS.md

@@ -0,0 +1,27 @@
+# interfaces/ — AGENTS.md
+
+## Purpose
+
+Reserved directory. Currently empty. Intended for shared interface or schema definitions that span multiple modules or external consumers.
+
+## Ownership
+
+Unassigned until content is added.
+
+## Local Contracts
+
+_None — directory is empty._
+
+Do not add content here speculatively. Add when a durable cross-module interface contract needs a home outside the Gradle module tree.
+
+## Work Guidance
+
+_None yet._
+
+## Verification
+
+_None yet._
+
+## Child DOX Index
+
+No child AGENTS.md (leaf directory).