feat: add configurable AgentRouter AI provider

internal/config/config_test.go

@@ -56,3 +56,29 @@ func TestLoadGrowthDefaultsAreCostSafe(t *testing.T) {
 		t.Fatalf("unsafe AI defaults: %+v", cfg)
 	}
 }
+
+func TestLoadSelectsAgentRouterProvider(t *testing.T) {
+	t.Setenv("DATABASE_URL", "postgresql://example")
+	t.Setenv("AI_PROVIDER", AIProviderAgentRouter)
+	t.Setenv("AGENTROUTER_API_KEY", "agentrouter-key")
+	t.Setenv("AGENTROUTER_BASE_URL", "https://agentrouter.example/v1")
+	t.Setenv("AGENTROUTER_MODEL", "gpt-test")
+
+	cfg, err := Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	provider := cfg.SelectedAIProvider()
+	if provider.Name != AIProviderAgentRouter || provider.APIKey != "agentrouter-key" || provider.BaseURL != "https://agentrouter.example/v1" || provider.Model != "gpt-test" {
+		t.Fatalf("SelectedAIProvider() = %+v", provider)
+	}
+}
+
+func TestLoadRejectsUnknownAIProvider(t *testing.T) {
+	t.Setenv("DATABASE_URL", "postgresql://example")
+	t.Setenv("AI_PROVIDER", "unknown")
+
+	if _, err := Load(); err == nil {
+		t.Fatal("Load() error = nil, want unknown provider error")
+	}
+}