docs: explain \$EDITOR variable and how to set it

\$EDITOR is blank by default on many systems -- running it without setting
it fails silently. Added a section before the git/gh instructions that
explains what it is, how to check if it's set, how to set it for a session
or permanently in .bashrc/.zshrc, and common editor options including
VS Code with --wait.

https://claude.ai/code/session_012eTokAaGiZo7aGt1T2W9BC

README.md

@@ -115,6 +115,39 @@ Look for any of these in the app's docs:
 
 ## Getting git and authenticating to GitHub
 
+### Set your preferred editor first
+
+The setup steps below use `$EDITOR` to open config files. It's a standard
+shell variable that points to whichever editor you like. If it isn't set,
+running `$EDITOR somefile` fails because the shell tries to execute an empty
+string.
+
+Check whether it's already set:
+
+```bash
+echo $EDITOR    # blank = not set
+```
+
+Set it for the current session, or make it permanent by adding the export
+to `~/.bashrc` (bash) or `~/.zshrc` (zsh):
+
+```bash
+# Pick one -- whatever you actually have installed:
+export EDITOR=nano
+export EDITOR=vim
+export EDITOR=micro
+export EDITOR="code --wait"   # VS Code (the --wait keeps the terminal paused until you close the file)
+
+# Make it permanent:
+echo 'export EDITOR=nano' >> ~/.bashrc && source ~/.bashrc
+# or for zsh:
+echo 'export EDITOR=nano' >> ~/.zshrc  && source ~/.zshrc
+```
+
+Once set, `$EDITOR somefile` opens that file in your chosen editor every time.
+
+### Install git
+
 If git isn't installed on the server yet:
 
 ```bash