Merge branch 'js/scalar'
Add pieces from "scalar" to contrib/.
* js/scalar:
scalar: implement the `version` command
scalar: implement the `delete` command
scalar: teach 'reconfigure' to optionally handle all registered enlistments
scalar: allow reconfiguring an existing enlistment
scalar: implement the `run` command
scalar: teach 'clone' to support the --single-branch option
scalar: implement the `clone` subcommand
scalar: implement 'scalar list'
scalar: let 'unregister' handle a deleted enlistment directory gracefully
scalar: 'unregister' stops background maintenance
scalar: 'register' sets recommended config and starts maintenance
scalar: create test infrastructure
scalar: start documenting the command
scalar: create a rudimentary executable
scalar: add a README with a roadmap