About mysqld monitoring & restarting if not running, etc

Check the MySQL error log to see if you can find out why it is crashing. Most likely it is something like the process using up too much RAM and getting killed by the OS. Do you see lots of swapping going on with your Linode in the manager?

If this is the issue, then you should look at tuning settings in your my.cnf file.

Monit is a nice lightweight monitoring daemon which will do what you're asking. Chances are good you can simply install it from your distro's repositories. Keep in mind it should not be a substitute for figuring out and fixing whatever is causing MySQL to crash.