dev/sg: do not use .zprofile, add compdef

Created by: bobheadxi

The only thing in my .zprofile was some brew stuff. I moved to .zshrc and it worked fine, so let's just put everything in .zshrc instead of splitting things.

compdef magically works for me, but some StackOverflow threads mention it might need to be explicitly initialized in .zshrc, so this PR adds it

Closes https://github.com/sourcegraph/sourcegraph/pull/36814

Test plan

Will be testing with @jwayman who is running into issues with the completions setup as is

Update - ran him through a setup on this branch and it worked