fix: setup and arty completion bugs

bin/setup:
- Fix Step 1 of 6 → Step 1 of 7
- exec zsh -l </dev/tty to avoid inheriting stale curl pipe on stdin
  (without /dev/tty the curl|sh pipe bytes re-run the script as zsh)
- Fall back to printing exec zsh hint when /dev/tty is unavailable
- Check completion file existence instead of trusting arty exit code

bin/arty:
- Fix cmd_completion returning exit 1 for zsh: last statement was
  [[ "$shell" == "bash" ]] which is false, changed to if/fi

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

bin/arty

@@ -1362,7 +1362,9 @@ cmd_completion() {
 
   echo "$content" > "$dest"
   log_success "$shell completion installed: $dest"
-  [[ "$shell" == "bash" ]] && log_info "Reload: source $dest"
+  if [[ "$shell" == "bash" ]]; then
+    log_info "Reload: source $dest"
+  fi
 }
 
 # USAGE AND HELP

bin/setup

@@ -99,7 +99,7 @@ detect_os() {
 
 # ─── Step 1: Prerequisites ────────────────────────────────────────────────────
 check_prerequisites() {
-  step_header "Step 1 of 6 — Prerequisites"
+  step_header "Step 1 of 7 — Prerequisites"
 
   detect_os
   log_info "OS detected: ${BOLD}$OS${NC}"
@@ -310,7 +310,8 @@ install_completion() {
 
   # arty completion zsh --install writes to the first writable fpath dir,
   # defaulting to ~/.zsh/completions/_arty (which .zshenv/oh-my-zsh picks up).
-  if ! (cd "$HOME" && "$arty_bin" completion zsh --install); then
+  (cd "$HOME" && "$arty_bin" completion zsh --install) || true
+  if [[ ! -f "$HOME/.zsh/completions/_arty" ]]; then
     log_warn "arty completion zsh --install failed — skipping."
   fi
 }
@@ -348,10 +349,13 @@ main() {
   install_completion   # Step 6
   post_install         # Step 7
 
-  # Hand off to zsh immediately — no need to exec zsh manually.
-  if command -v zsh >/dev/null 2>&1; then
-    exec zsh -l
+  # Hand off to a zsh login shell immediately.
+  # Read from /dev/tty so zsh gets the terminal, not the leftover pipe bytes
+  # from the curl|sh pattern (which still has the tail of this script buffered).
+  if command -v zsh >/dev/null 2>&1 && [[ -e /dev/tty ]]; then
+    exec zsh -l </dev/tty
   fi
+  echo -e "${YELLOW}[!]${NC} Start a new shell session: ${BOLD}exec zsh${NC}"
 }
 
 main "$@"