use same shell setup as in nix home manager

roles/pi_modern_shell_env/README.md

@@ -1,11 +1,19 @@
 # pi_modern_shell_env
 
 Installs modern CLI tooling (fish, ripgrep, fd, bat, fzf, zoxide, lsd,
-tmux) plus a starship prompt for the login user on `iot` Pi hosts, and
+tmux) plus an oh-my-posh prompt for the login user on `iot` Pi hosts, and
 makes fish the default login shell. Purely a convenience layer for
 interactive SSH sessions into otherwise single-purpose appliance boxes —
 does not affect any of the audio/sensor services these hosts run.
 
+The oh-my-posh theme (`files/ohmyposh.json`) and its fish vi-mode
+integration (`templates/config.fish.j2`) are copied from the same setup
+used in `~/nixconfig`'s home-manager config
+(`homeManagerModules/cli/modern-cli.nix`/`ohmyposh.json`), so the prompt
+looks and behaves the same on the desktop and on these Pis. If that
+nixconfig theme changes, re-sync `files/ohmyposh.json` here by hand — the
+two aren't linked.
+
 **Key vars:**
 
 - `pi_modern_shell_env_target_user` — user to configure (defaults to