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.

install.sh

@@ -269,9 +269,7 @@ for rel in \
     config/env/00-path.zsh \
     config/interactive/10-bootstrap.zsh \
     config/interactive/20-post-omz.zsh \
-    local/env.local.zsh.example \
-    local/login.local.zsh.example \
-    local/interactive.local.zsh.example
+    local.zsh.example
 do
     download_easyzsh_file "$rel"
 done