Add SIGHUP support to `dx serve` to force reload

## Feature Request

I'm using `dx serve` with Claude Code, but it's having trouble managing the dx serve process itself so I'm running `dx serve` in another terminal. This works reasonably well, except that sometimes when mid-edit the serve loop gets into a bad state and I need to tab over and hit `r` to force a rebuild.

I was hoping that CC could trigger that action itself by sending a `SIGHUP` signal to dx (e.g. by using `pkill -HUP dx` but currently that actually just kills dx serve.

## Implement Suggestion

As far as prior art, nginx is a good example where SIGHUP forces a reload of nginx.conf. As a first pass, having SIGHUP behave just like pressing `r` seems like a good start.