fix: unquote PAYLOAD heredoc so runtime vars are expanded

The inner PAYLOAD heredoc had a quoted delimiter ('PAYLOAD') which
prevented runtime variable expansion. Changed to unquoted (PAYLOAD)
so $AGENT_KEY, $CPU, etc. are correctly expanded at runtime.

The double-escaping works as:
  install time (outer EOF):  $AGENT_KEY  →   (in file)
  runtime (inner PAYLOAD):     →  abc123... (actual value)

install/agent-install.sh

@@ -62,7 +62,7 @@ collect_metrics() {
 }
 
 push_metrics() {
-    payload=\$(cat << 'PAYLOAD'
+    payload=\$(cat << PAYLOAD
 {
     "agent_key": "\$AGENT_KEY",
     "cpu": \$CPU,