Graceful warning when trying to kill a worker that's running a stage

Trap the first Ctrl-C to workproc and show a warning if the worker is running a job.

On the second Ctrl-C, gracefully execute a cleanup phase to write back changes from the `make` stage.

Start at the workproc entry point and trace how worker jobs and the `make` stage respond to Ctrl-C. The issue names no files or tests, so first locate the signal-handling and cleanup code, then verify that the first interrupt warns during a job and the second writes back changes before exiting.

Written by the indexing model from the issue text.