macOS Mojave CalendarAgent runaway process
Well, it appears that somehow the upgrade must have left the calendar info in a weird state, because this seems to have fixed it:


Disable all Google calendar accounts (Calendar.app -> Preferences... -> Accounts).
Wait for the CalendarAgent process to subside. Eventually it will; use Activity Monitor to check. A reboot may help at this point.
Enable each Google calendar account, one by one: every time one gets enabled, the CalendarAgent process will get excited for a while. Wait for it to idle before enabling the next account.