feat: one-liner installer + setup install ping

- install.sh: curl-pipe-bash installer with prereq checks (git, bun),
  upgrade detection (git pull if already installed), transparency note
  about update-check pings
- setup: add install ping at end (gstack-update-check --force) to
  register day-zero installs in Supabase
- Install ping only in setup (not install.sh) to avoid double-counting
  (Codex review fix #7)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

setup

@@ -434,3 +434,10 @@ if [ ! -d "$HOME/.gstack" ]; then
   echo "  Welcome! Run /gstack-upgrade anytime to stay current."
 fi
 rm -f /tmp/gstack-latest-version
+
+# 9. Install ping (best-effort, non-blocking)
+# Fires update-check with --force to register this install in Supabase.
+# Sends only: version, OS, random UUID. No usage data.
+if [ -x "$SOURCE_GSTACK_DIR/bin/gstack-update-check" ]; then
+  "$SOURCE_GSTACK_DIR/bin/gstack-update-check" --force 2>/dev/null &
+fi