Add Mage build system

Set up magefiles/ directory with targets for test, coverage,
lint, and build commands. Uses github.com/magefile/mage v1.15.0.

magefiles/magefile.go

@@ -0,0 +1,63 @@
+//go:build mage
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"runtime"
+
+	"github.com/magefile/mage/sh"
+)
+
+// Test runs all tests with race detection and coverage.
+func Test() error {
+	fmt.Println("Running tests with race detection and coverage...")
+	return sh.RunV("go", "test", "-race", "-coverprofile=coverage.out", "./...")
+}
+
+// Coverage runs tests and opens the HTML coverage report in a browser.
+func Coverage() error {
+	if err := Test(); err != nil {
+		return err
+	}
+
+	fmt.Println("Generating HTML coverage report...")
+	if err := sh.RunV("go", "tool", "cover", "-html=coverage.out", "-o", "coverage.html"); err != nil {
+		return err
+	}
+
+	fmt.Println("Opening coverage report...")
+	return openBrowser("coverage.html")
+}
+
+// Lint runs golangci-lint.
+func Lint() error {
+	fmt.Println("Running golangci-lint...")
+	return sh.RunV("golangci-lint", "run", "./...")
+}
+
+// Build verifies the module compiles.
+func Build() error {
+	fmt.Println("Verifying module compiles...")
+	return sh.RunV("go", "build", "./...")
+}
+
+// openBrowser opens the specified file in the default browser.
+func openBrowser(file string) error {
+	var cmd *exec.Cmd
+	switch runtime.GOOS {
+	case "linux":
+		cmd = exec.Command("xdg-open", file)
+	case "darwin":
+		cmd = exec.Command("open", file)
+	case "windows":
+		cmd = exec.Command("rundll32", "url.dll,FileProtocolHandler", file)
+	default:
+		return fmt.Errorf("unsupported platform: %s", runtime.GOOS)
+	}
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Start()
+}