feat(setup): rewrite setup-mac.sh around Brewfiles with idempotent steps

The old script stopped on a syntax error (unclosed if blocks) and had
drifted from the machine: it installed Anaconda, AeroSpace and Oh My
Posh, and linked nothing.

- Eight named steps (xcode homebrew brew links shell runtimes ai apps)
  that check what is in place first, run on their own by name, and
  preview with --dry-run; a failed step is reported while the rest run
- Brewfile holds the CLI tools, fonts and terminals, Brewfile.apps the
  GUI casks and App Store apps; installs never upgrade existing packages
  and skip apps already installed outside Homebrew
- links symlinks the configs and .local/bin scripts, backing up what it
  replaces, and fetches the nvim submodule only on a fresh clone
- Guard .zshrc's ~/.deno/env source for machines without it
- Document the steps in the README

.zshrc

@@ -322,7 +322,7 @@ if [ -d "$HOME/neovim/bin" ]; then
     export PATH="$HOME/neovim/bin:$PATH"
 fi
 
-. "$HOME/.deno/env"
+[ -f "$HOME/.deno/env" ] && . "$HOME/.deno/env"
 
 export NVM_DIR="$HOME/.config/nvm"
 [ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"  # This loads nvm