fix(shell): use single ~/.config/zsh/local.zsh for machine local

Replace the local/ directory of split env/login/interactive files with
one always-on local.zsh sourced from .zshenv. Deploy migrates old
fragments and never overwrites local.zsh.

.zshenv

@@ -1,5 +1,5 @@
 # easyzsh stub: every zsh loads this. Keep it thin; logic lives under ~/.config/zsh/.
-# Machine-local early env: ~/.config/zsh/local/env.local.zsh (never overwritten).
+# Machine-local overrides: ~/.config/zsh/local.zsh (never overwritten by deploy).
 EASYZSH_HOME="${XDG_CONFIG_HOME:-$HOME/.config}/zsh"
 
 if [[ -d "$EASYZSH_HOME/env" ]]; then
@@ -10,9 +10,9 @@ if [[ -d "$EASYZSH_HOME/env" ]]; then
   unset _easyzsh_f
 fi
 
-if [[ -f "$EASYZSH_HOME/local/env.local.zsh" ]]; then
+if [[ -f "$EASYZSH_HOME/local.zsh" ]]; then
   # shellcheck disable=SC1090
-  . "$EASYZSH_HOME/local/env.local.zsh"
+  . "$EASYZSH_HOME/local.zsh"
 elif [[ -f "$HOME/.config/zshenv.local" ]]; then
   # Legacy path from earlier easyzsh layouts.
   # shellcheck disable=SC1090