Add voipms-trunk service: US-only outbound PSTN, max 3 concurrent calls

Adds a VoIP.ms SIP trunk on top of asterisk-digital-ocean: IP-authenticated
trunk (no password stored), NANP-only outbound dialplan, a global 3-call
concurrent cap via GROUP()/GROUP_COUNT(), and inbound routing to one
extension. Config lives in its own include files rather than being
appended directly to pjsip.conf/extensions.conf, since Easy Asterisk fully
regenerates both from its own internal state — the includes are patched
into the vendor's generator functions so they survive that regeneration.

Wires the new service into setup.sh's is_installed() and README's services
table, and updates docs/pstn-calling-voipms-plan.md to reflect what's now
implemented vs. still open (spend/volume alerting, live-account
verification).

setup.sh

@@ -89,6 +89,7 @@ is_installed() {
         sync-cc) [ -f "$ACTUAL_HOME/sync-cc/sync_cc.py" ] ;;
         sky-cam) [ -d "$ACTUAL_HOME/sky-cam/.git" ] ;;
         sky-cam-frigate) [ -d "$ACTUAL_HOME/sky-cam/.git" ] && [ -f "$ACTUAL_HOME/sky-cam/frigate-retime.sh" ] ;;
+        voipms-trunk) [ -f "$DOCKER_DIR/asterisk-digital-ocean/config/asterisk/voipms-trunk-pjsip.conf" ] ;;
         ssh-config) false ;;   # repeatable management tool, never shows [installed]
         *) [ -e "$DOCKER_DIR/$1" ] ;;
     esac