report what a section actually installed, not what it was asked for

The summary claimed credit for everything in a section's list, including the
packages it had just decided to leave alone — so a run that installed nothing
still ended with "Command-line tools: lazydocker lazygit starship fastfetch".
The announce above it said "nothing, all present" in the same breath.

pkg_install and tools_install now record LAST_INSTALLED and LAST_KEPT, and
summarise_last turns those into one honest line:

  Core packages installed: btop tmux (17 already present)
  Command-line tools: already present, nothing installed

Verified all three shapes — everything present, nothing present, and mixed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/setup/machine-setup/lib/tools.sh

@@ -113,6 +113,9 @@ tools_install() {
     if tool_is_installed "$tool"; then present+=("$tool"); else missing+=("$tool"); fi
   done
 
+  LAST_INSTALLED=("${missing[@]}")
+  LAST_KEPT=("${present[@]}")
+
   info "$label — installs what is missing, keeps what you already have"
   ((${#present[@]})) && echo "     already here: ${present[*]}"