After I've customized a template, I'd like to add it to /templates/ so that people can copy it to modify themselves (and can't delete it). But when I place my template in the /templates directory, it doesn't appear in the template editor.

If I place it in the /upload/templates/ folder of course it does, but then people who have permissions to manage templates can delete it.

I'm using the latest version of LS 2.65.0 (170502)

Any advice?

For now you should hack the core code to add your template to standard templates:
github.com/LimeSurvey/LimeSurvey/blob/ma...ls/Template.php#L280

We're about to do a new release this week. I will modify the code so the list of standard templates will come from the configuration file (so you can modify it without hacking the code).

Excellent! Thanks for including that in the next release