feat: add system package upgrade to stage 00

- Add PKG_UPGRADE variable to lib/distro.sh:
  - Debian: sudo apt upgrade -y
  - Fedora: sudo dnf upgrade -y
- Export PKG_UPGRADE so stage scripts can use it
- Add upgrade step to 00-envcheck.sh after the existing cache update
  - Errors are non-fatal (|| warn) to handle fresh installs gracefully

stages/00-envcheck.sh

@@ -36,4 +36,8 @@ fi
 info "Updating package cache (first run)..."
 $PKG_UPDATE 2>/dev/null || warn "Package cache update had issues."
 
+# ---- System upgrade ----
+info "Upgrading all system packages..."
+$PKG_UPGRADE 2>/dev/null || warn "System upgrade had issues (this is okay on a fresh install)."
+
 ok "Stage 00 complete."