Add newcomer guide, agent guide, and sample app files

DIACHRON.md explains the framework to newcomers joining a
diachron-based project.  diachron/AGENTS.md helps AI coding
agents work with the framework conventions and commands.
backend/types.ts and backend/services.ts are sample starting
points for application-specific types and services.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services.ts

@@ -0,0 +1,15 @@
+// This is a sample file provided by diachron.  You are encouraged to modify it.
+
+// Application services go here.  A service encapsulates a capability that
+// handlers depend on: database queries, external API calls, business logic
+// that doesn't belong in a handler.
+//
+// The framework provides core services via `core` (from ./diachron/core):
+// core.database, core.logging, core.misc, etc.  This file is for your
+// application's own services.
+
+import { core } from "./diachron/core";
+
+const db = core.database.db;
+
+export { db };