Well I would ride it for sure, but I'm famously cavalier about that. If you are especially risk-averse, or if just thinking about it will ruin your enjoyment of the ride, then replace it.
I would not attempt to repair it, because that will be cosmetic only. It will not make it less likely to break there.

Factors in your favor:
LHTs are robust frames, not superlight, and that is such a visible spot that it'll be easy to see any crack, should one start. Fatigue cracks in steel frames almost always grow slowly and give lots of warning.

Just don't try to hide the spot with tape or painter's putty or what-have-you, we need to be able to see a crack, should one start. In my opinion, it is very unlikely to crack, ever.