fix: reset outboundPending on call end to prevent inbound poisoning

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

src/state/sip-manager.ts

@@ -75,6 +75,7 @@ export function connectSip(config: SIPConfig): void {
             // Reset outbound flag when call ends
             if (state === 'ended' || state === 'failed') {
                 outboundActive = false;
+                outboundPending = false;
             }
         },
     );