If I move to Panama permanently full time what do I have to do to avoid paying U.S. federal income tax on my federal govt pension?
The only way not to have to file US Tax Returns is to give up your US citizenship or residency.
According to the IRS and related questions on this site:
Your worldwide income is subject to U.S. income tax, regardless of where you reside.
There may be possibilities depending on tax treaties in place to avoid paying taxes both in Panama and the US but you will need to look into it or consult an accountant.