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/types.ts

@@ -0,0 +1,8 @@
+// This is a sample file provided by diachron.  You are encouraged to modify it.
+
+// Application-specific types go here.  Framework types (Call, Result, Route,
+// Handler, etc.) are defined in ./diachron/types and should be imported from
+// there.
+//
+// This file is for your domain types: the nouns and shapes that are specific
+// to your application.