Add opt-in auto-update from Gitea releases

The program now lives in /usr/local/lib/bamboo and is owned by its service user so it can replace itself.

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

.env.example

@@ -9,3 +9,7 @@ BAMBOO_TOKEN=
 
 # Folder for this agent's WireGuard key.
 #BAMBOO_DATA=./bamboo-data
+
+# 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.
+#BAMBOO_AUTO_UPDATE=1