fix: exec zsh -l at end of setup, remove exec zsh hint

The script already sets zsh as default shell via chsh. Now it also
hands off to a zsh login shell immediately at the end so the user
doesn't need to run exec zsh manually.

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

bin/setup

@@ -322,17 +322,14 @@ post_install() {
   echo
   echo -e "${GREEN}${BOLD}Setup complete.${NC} Next steps:"
   echo
-  echo -e "  ${CYAN}1.${NC} Start a new shell session:"
-  echo -e "       ${BOLD}exec zsh${NC}"
-  echo
-  echo -e "  ${CYAN}2.${NC} First time on this machine? Configure the Powerlevel10k prompt:"
+  echo -e "  ${CYAN}1.${NC} First time on this machine? Configure the Powerlevel10k prompt:"
   echo -e "       ${BOLD}p10k configure${NC}"
   echo
-  echo -e "  ${CYAN}3.${NC} Generate an SSH key for git push access to dev.pivoine.art:"
+  echo -e "  ${CYAN}2.${NC} Generate an SSH key for git push access to dev.pivoine.art:"
   echo -e "       ${BOLD}arty ssh:keygen${NC}"
   echo -e "     Then add the public key at ${BLUE}https://dev.pivoine.art${NC}"
   echo
-  echo -e "  ${CYAN}4.${NC} Switch the dotfiles remote from HTTPS to SSH (after key is set up):"
+  echo -e "  ${CYAN}3.${NC} Switch the dotfiles remote from HTTPS to SSH (after key is set up):"
   echo -e "       ${BOLD}git -C ~ remote set-url origin ssh://git@dev.pivoine.art/valknar/home.git${NC}"
   echo
 }
@@ -350,6 +347,11 @@ main() {
   install_deps         # Step 5
   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
+  fi
 }
 
 main "$@"