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>

file-list

@@ -2,6 +2,7 @@
 
 .gitignore
 .go-version
+DIACHRON.md
 backend/.gitignore
 backend/.npmrc
 backend/app.ts
@@ -17,13 +18,16 @@ backend/package.json
 backend/pnpm-workspace.yaml
 backend/routes.ts
 backend/run.sh
+backend/services.ts
 backend/show-config.sh
 backend/tsconfig.json
+backend/types.ts
 backend/watch.sh
 bootstrap.sh
 cmd
 develop
 diachron
+diachron/AGENTS.md
 file-list
 logger
 master