Save after POSIX signal

Description of the problem or steps to reproduce

This is not a bug - it's a feature suggestion. I'm using micro as my always-open notes editor, holding a TODO list and many other little notes. Some of these notes gets edited elsewhere too - for example I have a script that prepends today's date every morning to the top of my TODO file. When this happens, micro asks if I want to reload the file, which puts me in a tricky situation: If I don't reload, I don't see the new day's date, but if I do reload I risk that maybe I have some unsaved changes.

I can't really turn auto-save because it previosuly caused some issues with syncing my notes.

I was wondering if perhaps we could use SIGUSR1 or alike to signal the micro process to trigger a save. If we could do that, all my scripts that edit the files outside of micro could trigger a save before editing the files and I can rest assure no data is lost.

Note that this doesn't really have to be through a POSIX signal - it could be any programmatic way to trigger a save from outside - I just thought a signal might be easier to implement and is already some kind of a universal standard.

Thank you for micro!

Specifications

Commit hash: cc994b62
OS: Arch Linux
Terminal: Alacritty

The issue names no files, tests, or entry points. Start by locating micro’s signal handling and save paths, then clarify the external trigger and behavior around unsaved changes; done means the chosen mechanism reliably saves before external edits and has coverage for the relevant cases.