How to run a shell script from an AppleScript

I don't know what the exit status of 78 means, but I would not expect this script to do anything detectable when run as a launch agent, because its output is not directed anywhere. When you run it from Terminal, its output is attached to that Terminal window, so that's where the "hello" appears. But launchd has no connection to Terminal (or any other relevant output target), so the "hello" gets discarded. If you want to check whether it's running, you can direct its outputs (both standard output and error output) to files by adding these keys to the .plist (and then unloading and reloading it):

<key>StandardOutPath</key>
<string>/Users/yangyy/hello-output.txt</string>
<key>StandardErrorPath</key>
<string>/Users/yangyy/hello-errors.txt</string>

Now, it's also possible that launchd is having some other problem even starting the script. To see if launchd itself is reporting any problems, look in the system log file (tail -f /var/log/system.log or run /Applications/Utilities/Console.app), then try to unload+reload the agent and see if anything relevant appears in the log.