Adjust install, file-copy, and theme logic after first run on arch linux

docs/post-run.md

@@ -1,5 +1,14 @@
 # Manual steps needed after dotfile copy and/or installs
 
+## all systems
+
+- shell
+    - check user's shell using `echo $SHELL` or otherwise
+    - if not the desired shell:
+        - `chsh -l` to see options; if target shell isn't listed, then add it
+          to `/etc/shells`
+        - then change the shell with `chsh -s <path to target shell>`
+
 ## macOS
 
 - aerospace