cli: Fix incorrect prompt setting for Zsh

It incorrectly always ended up using the Bash method for setting the
prompt, and in the Zsh prompt setting, it didn't correctly mark which
parts of the prompt are zero-length.

cli/cloe_launch/exec.py

@@ -431,20 +431,10 @@ def _write_prompt_sh(self) -> None:
         prompt_sh_file = self.runtime_dir / "prompt.sh"
         prompt_sh_data = textwrap.dedent(
             """\
-            CLOE_PROMPT="\u001b[2m[cloe-shell]\u001b[0m"
-
-            prompt_cloe() {
-                if [[ -n $CLOE_SHELL ]]; then
-                    PROMPT="%{%F{242}%}[cloe-shell]%f ${PROMPT}"
-                fi
-            }
-
-            CURRENT_SHELL=$(basename $0)
-            if [[ $CURRENT_SHELL = "zsh" ]]; then
-                autoload -Uz add-zsh-hook
-                add-zsh-hook precmd prompt_cloe
+            if [[ -n $ZSH_VERSION ]]; then
+                export PS1="%{%F{242}%}[cloe-shell]%{%f%} ${PS1}"
             else
-                export PS1="$CLOE_PROMPT $PS1"
+                export PS1="\u001b[2m[cloe-shell]\u001b[0m $PS1"
             fi
             """
         )