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/machine-setup.sh

@@ -91,8 +91,8 @@ if ! skip; then
   # shellcheck disable=SC2046  # word splitting is how the list is passed
   pkg_install "Core packages" $(pkgs_core)
 
+  summarise_last "Core packages"
   ok "System updated and core packages in place"
-  SUMMARY+=("System packages updated, core packages installed")
   step_ok
 fi
 
@@ -109,8 +109,7 @@ step "Command-line tools"
 if ! skip; then
   # shellcheck disable=SC2046  # word splitting is how the list is passed
   tools_install "Command-line tools" $(tools_default)
-
-  SUMMARY+=("Command-line tools: $(tools_default)")
+  summarise_last "Command-line tools"
   step_ok
 fi