Refactor installation and runner scripts for enhanced usability and security

- Updated the installation script to clarify that production wrappers do not include fixed test secrets, improving user guidance for local testing.
- Modified the jiggablend-manager and jiggablend-runner scripts to remove fixed test configurations, emphasizing the use of local test credentials via `make init-test`.
- Enhanced error handling in the runner script to ensure that an API key is provided, improving security and user feedback.
- Added new flags and options for the runner, including sandboxing capabilities and GPU ray tracing control, enhancing flexibility for users.
- Improved README documentation to reflect changes in usage and configuration, ensuring users have clear instructions for setup and execution.

internal/runner/tasks/exr_merge.go

@@ -0,0 +1,46 @@
+package tasks
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// mergeEXRFiles averages multiple linear EXR renders into one (Monte Carlo accumulation).
+func mergeEXRFiles(output string, inputs []string) error {
+	if len(inputs) == 0 {
+		return fmt.Errorf("no input EXR files to merge")
+	}
+	if len(inputs) == 1 {
+		return copyFile(inputs[0], output)
+	}
+
+	args := append([]string{}, inputs...)
+	args = append(args, "-evaluate-sequence", "mean", output)
+
+	if path, err := exec.LookPath("magick"); err == nil {
+		cmd := exec.Command(path, args...)
+		if out, err := cmd.CombinedOutput(); err != nil {
+			return fmt.Errorf("magick merge failed: %w (%s)", err, strings.TrimSpace(string(out)))
+		}
+		return nil
+	}
+
+	if path, err := exec.LookPath("convert"); err == nil {
+		cmd := exec.Command(path, args...)
+		if out, err := cmd.CombinedOutput(); err != nil {
+			return fmt.Errorf("convert merge failed: %w (%s)", err, strings.TrimSpace(string(out)))
+		}
+		return nil
+	}
+
+	return fmt.Errorf("ImageMagick (magick or convert) required to merge batched EXR renders")
+}
+
+func copyFile(src, dst string) error {
+	cmd := exec.Command("cp", "-f", src, dst)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("copy %s to %s: %w (%s)", src, dst, err, strings.TrimSpace(string(out)))
+	}
+	return nil
+}