SSH key issue with use of GitHub repository and desktop client for pull/commit/push (SourceTree) on MAC

After a number of years away from a GitHub type work environment I started to use RStudio for some data analysis and l am loving it. Recently I decided to start using GitHub again to store/share my code with folks that I'm working on things with. I saw some posts about SourceTree and decided to give it a go, having historically just used the Terminal. First couple days were soooo sweet, then one day I couldn't commit anything 'Permissions Error', What!

Turns out Apple doesn’t seem to remember SSH keys between reboots, OMG!
Fix here; specifically I used what is noted as Method 1: Add all known keys to the SSH agent.

On my way there I got really familiar with various GitHub references;

Reasonably good reminder of Vi commands

I thought it might prove useful to capture here to save others from hours of searching and the brilliant pain of such a stupid issue taking so much time.