What voltage are AGM battery capacity ratings supposed to be computed at?
It depends. Manufacturers use the C20 (20-hour), C10 (10-hour) or C5 (5-hour) capacity ratings as a standard way to express the total amount of Ah a battery can deliver when completely discharged in a given time, under given circumstances.

Note that this value says nothing about the battery voltage, only the current delivered over time. The battery voltage during this time will vary between about 12.7 Volts (fully charged) to 10.8 Volts (empty). The detailed behaviour should be looked up in the manufacturer's datasheet.

Generally, the slower an AGM battery (or any lead acid battery) is discharged, the higher the total amount of Ah it can deliver.

I.e. the total amount of energy delivered by a battery is highly dependent on the circumstances.