docs(config): document agent backend routing and add schema guard test

src/config/schema.test.ts

@@ -418,6 +418,19 @@ describe('configSchema — agent_configs', () => {
     expect(result.agent_configs.assistant.tool_profile).toBe('messaging');
     expect(result.agent_configs.coder.sandbox).toBe(true);
   });
+
+  it('rejects invalid backend name in agent config', () => {
+    expect(() => configSchema.parse({
+      ...minimalConfig,
+      agent_configs: {
+        assistant: {
+          system_prompt: 'You are helpful.',
+          model_tier: 'default',
+          backend: 'unknown_backend',
+        },
+      },
+    })).toThrow();
+  });
 });
 
 describe('configSchema — backends', () => {