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.

apps/desktop/AGENTS.md

@@ -0,0 +1,27 @@
+# apps/desktop
+
+## Purpose
+
+Stub module reserved for a future desktop GUI application. Not yet implemented.
+
+## Ownership
+
+`apps/desktop/` — currently only a placeholder `Main.kt`.
+
+## Local Contracts
+
+None defined yet.
+
+## Work Guidance
+
+Do not add functionality here until the desktop epic is planned. Follow Kotlin rules in root CLAUDE.md when work begins.
+
+## Verification
+
+```
+./gradlew :apps:desktop:test --rerun-tasks
+```
+
+## Child DOX Index
+
+No child AGENTS.md (leaf module).