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/sandbox/none.go

@@ -0,0 +1,18 @@
+package sandbox
+
+import "os/exec"
+
+type noneWrapper struct{}
+
+func (w *noneWrapper) Name() string { return BackendNone }
+
+func (w *noneWrapper) Available() error { return nil }
+
+func (w *noneWrapper) Wrap(spec Spec) (*exec.Cmd, error) {
+	cmd := exec.Command(spec.BlenderBinary, spec.Args...)
+	cmd.Dir = spec.WorkDir
+	if len(spec.Env) > 0 {
+		cmd.Env = spec.Env
+	}
+	return cmd, nil
+}