feat: add slog structured logging

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

pkg/logger/logger_test.go

@@ -0,0 +1,39 @@
+package logger
+
+import (
+	"testing"
+
+	"codespace/pkg/config"
+)
+
+func TestNewJSONInfoLogger(t *testing.T) {
+	lg, err := New(config.LogConfig{Level: "info", Format: "json"})
+	if err != nil {
+		t.Fatalf("New returned error: %v", err)
+	}
+	if lg == nil {
+		t.Fatal("logger is nil")
+	}
+}
+
+func TestNewTextDebugLogger(t *testing.T) {
+	lg, err := New(config.LogConfig{Level: "debug", Format: "text"})
+	if err != nil {
+		t.Fatalf("New returned error: %v", err)
+	}
+	if lg == nil {
+		t.Fatal("logger is nil")
+	}
+}
+
+func TestNewRejectsInvalidLevel(t *testing.T) {
+	if _, err := New(config.LogConfig{Level: "verbose", Format: "json"}); err == nil {
+		t.Fatal("expected invalid level error")
+	}
+}
+
+func TestNewRejectsInvalidFormat(t *testing.T) {
+	if _, err := New(config.LogConfig{Level: "info", Format: "xml"}); err == nil {
+		t.Fatal("expected invalid format error")
+	}
+}