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.

local.zsh.example

@@ -0,0 +1,6 @@
+# Copy to ~/.config/zsh/local.zsh (deploy never overwrites local.zsh).
+# Sourced from ~/.zshenv after managed env modules — every zsh sees this,
+# including non-interactive agent shells.
+#
+# . "$HOME/.cargo/env"
+# source ~/.orbstack/shell/init.zsh 2>/dev/null || :