All:
For reference and the quick aid of future Google searchers, I figured this problem out, and it was a bit of a *headdesk* moment for me.
For starters, I mistyped the error message, glossing over an important detail - the actual error message was:
Git commit failed: /storage/sdcard0/AppProjects/eulerrunner/res/values/strings.xml not in repository /mnt/sdcard/AppProjects/eulerrunner
Of course, now the message is telling me exactly what is wrong. When I cloned the Git repo, it seems Git resolved the symlink, and AIDE did not (or vice versa, I can't remember which way the symlink points). When I did the commit, the paths didn't match. So, it's not a bug, really, just a confusion with symlinked folders. However, I figure someone else has to have run into this.
All I had to do was browse through the file manager from the symlinked /mnt/sdcard/AppProjects folder to /storage/sdcard0/AppProjects, and ran the commit from there. It worked like a charm, and I'm hacking away on my tablet again.
Take care,