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.

.zshrc

@@ -1,6 +1,6 @@
 # easyzsh stub: interactive shells only. Keep it thin; logic lives under ~/.config/zsh/.
 # Personal interactive overlays: ~/.config/zshrc/*.zsh (before oh-my-zsh).
-# Machine-local interactive: ~/.config/zsh/local/interactive.local.zsh (after omz).
+# Machine-local overrides: ~/.config/zsh/local.zsh (loaded from .zshenv for every zsh).
 EASYZSH_HOME="${XDG_CONFIG_HOME:-$HOME/.config}/zsh"
 
 if [[ -d "$EASYZSH_HOME/interactive" ]]; then
@@ -10,8 +10,3 @@ if [[ -d "$EASYZSH_HOME/interactive" ]]; then
   done
   unset _easyzsh_f
 fi
-
-if [[ -f "$EASYZSH_HOME/local/interactive.local.zsh" ]]; then
-  # shellcheck disable=SC1090
-  source "$EASYZSH_HOME/local/interactive.local.zsh"
-fi