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Hi. What was the watering schedule ?
As all branch seem affected equally, you have a soil fungus that is probably caused by overwatering (this cause the wilt). The rocks at the bottom may prevent surface drying and you may also lack proper drainage.
A dilluted bleach treatment sometime help but only for small pots. As you are in the main soil, erradication of the pathogene will not be possible. The best is to drastically cut on water for now. Sorry that i don't have much better news, chance for survival are about 10% from be pic i see.
If your garden store carry some systemic fungicides, you could try that also.
As it is the pathogen is already in the stem. A systemic fungicides canfollow them there but only if taken early because they clog the stem quite fast and internal water can't bring it up anymore.