feat: add slog structured logging

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkg/config/config_test.go

@@ -94,3 +94,52 @@ func TestLoadRejectsInvalidDuration(t *testing.T) {
 		t.Fatal("expected invalid duration error")
 	}
 }
+
+func TestDefaultIncludesLogConfig(t *testing.T) {
+	cfg := Default()
+	if cfg.Log.Level != "info" {
+		t.Fatalf("Log.Level = %q, want info", cfg.Log.Level)
+	}
+	if cfg.Log.Format != "json" {
+		t.Fatalf("Log.Format = %q, want json", cfg.Log.Format)
+	}
+}
+
+func TestLoadParsesLogConfigFromYAML(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+	data := []byte(`log:
+  level: "debug"
+  format: "text"
+`)
+	if err := os.WriteFile(path, data, 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load returned error: %v", err)
+	}
+	if cfg.Log.Level != "debug" {
+		t.Fatalf("Log.Level = %q, want debug", cfg.Log.Level)
+	}
+	if cfg.Log.Format != "text" {
+		t.Fatalf("Log.Format = %q, want text", cfg.Log.Format)
+	}
+}
+
+func TestLoadAppliesLogEnvironmentOverrides(t *testing.T) {
+	t.Setenv("CODESPACE_LOG_LEVEL", "warn")
+	t.Setenv("CODESPACE_LOG_FORMAT", "json")
+
+	cfg, err := Load(filepath.Join(t.TempDir(), "missing.yaml"))
+	if err != nil {
+		t.Fatalf("Load returned error: %v", err)
+	}
+	if cfg.Log.Level != "warn" {
+		t.Fatalf("Log.Level = %q, want warn", cfg.Log.Level)
+	}
+	if cfg.Log.Format != "json" {
+		t.Fatalf("Log.Format = %q, want json", cfg.Log.Format)
+	}
+}