zsh: print a user-facing banner when the bootstrap fires

The bootstrap silently installs tools for ~30s+ on a version bump or
first login, which looked like an unexplained hang. Now prints a
banner explaining what is happening, that it may take a minute, and
that subsequent runs are a no-op. The box is built dynamically so the
right edge stays aligned regardless of the version string length.
Also clarifies the trigger comment in zshrc.

zsh/zshrc

@@ -32,7 +32,10 @@ install_opencode(){
   curl -fsSL https://opencode.ai/install | bash
 }
 
-# Bootstrap: download and run once per bootstrap version
+# Bootstrap: download and run once per bootstrap version. Installs missing CLI
+# tools and oh-my-zsh/plugins across distros; prints a banner so the user knows
+# what the activity is. Re-runs only when ZSHRC_BOOTSTRAP_VERSION is bumped or
+# BOOTSTRAP=true is set.
 if [[ ! -f "$ZSHRC_BOOTSTRAP" || "$(cat "$ZSHRC_BOOTSTRAP" 2>/dev/null)" != "$ZSHRC_BOOTSTRAP_VERSION" || "$BOOTSTRAP" == "true" ]]; then
   tmp=/tmp/.zshrc_bootstrap.$$
   curl -fsSL --connect-timeout 5 --max-time 10 -o "$tmp" "$ZSHRC_BOOTSTRAP_URL" 2>/dev/null