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/packages.sh

@@ -29,6 +29,13 @@
 [[ -n "${MACHINE_SETUP_PACKAGES_LOADED:-}" ]] && return 0
 MACHINE_SETUP_PACKAGES_LOADED=1
 
+# What the last pkg_install/tools_install actually put on the machine, as opposed
+# to what it was asked for. Read by the caller to write an honest summary line:
+# without it every section reports its whole list as installed, including the
+# packages it deliberately left alone.
+LAST_INSTALLED=()
+LAST_KEPT=()
+
 # -----------------------------------------------------------------------------
 # The sections
 # -----------------------------------------------------------------------------
@@ -152,6 +159,9 @@ pkg_install() {
     if pkg_is_installed "$pkg"; then present+=("$pkg"); else missing+=("$pkg"); fi
   done
 
+  LAST_INSTALLED=("${missing[@]}")
+  LAST_KEPT=("${present[@]}")
+
   info "$label — installs what is missing, keeps what you already have"
   ((${#present[@]})) && echo "     already here: ${present[*]}"
 
@@ -163,3 +173,16 @@ pkg_install() {
   echo "     to install:   ${missing[*]}"
   pkg_install_now "${missing[@]}"
 }
+
+# One summary line describing what a section actually did, from LAST_INSTALLED
+# and LAST_KEPT. Call straight after pkg_install or tools_install.
+summarise_last() {
+  local label="$1"
+  if ((${#LAST_INSTALLED[@]} == 0)); then
+    SUMMARY+=("$label: already present, nothing installed")
+  elif ((${#LAST_KEPT[@]} == 0)); then
+    SUMMARY+=("$label installed: ${LAST_INSTALLED[*]}")
+  else
+    SUMMARY+=("$label installed: ${LAST_INSTALLED[*]} (${#LAST_KEPT[@]} already present)")
+  fi
+}