feat: add bin/setup bootstrap script and update README installation docs

- bin/setup: curl-pipeable installer — re-execs with bash, detects OS,
  installs git (apt/xcode-select), clones dotfiles into ~, runs arty deps,
  installs arty zsh completion, prints next steps; fully idempotent
- README.md: replace Quick Start with Installation one-liner, add step table,
  After Installation and Manual Setup sections; fix stale ~/scripts reference
- .gitignore: whitelist bin/setup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -20,6 +20,7 @@
 !bin/arty
 !bin/stacks
 !bin/README.md
+!bin/setup
 !.vscode/
 !.vscode/settings.json
 !.vscode/keybindings.json