Use the auto-updating Blacktip, Bamboo and Reef; add the -auto-update option

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.env.example

@@ -38,3 +38,7 @@ HAMMERHEAD_MASTER_KEY=
 # Token from Sites -> Show command.
 #BAMBOO_HAMMERHEAD=https://hammerhead.example.com
 #BAMBOO_TOKEN=
+
+# Check for new releases every few hours and update and restart by itself (Linux and macOS).
+# setup.sh turns this on; remove it to update by hand.
+#HAMMERHEAD_AUTO_UPDATE=1