Add read-only observer URL with 'whose turn' indicator

Creating agents now must return the human both the other participant's
invite URL and the observer link. Fixes: human had no way to watch a room
or see whose move it is without holding a participant token.

examples/two-agents.sh

@@ -29,11 +29,13 @@ CREATED=$(curl -sf -X POST -H 'content-type: application/json' -d '{
 ROOM=$(echo "$CREATED" | node -e 'let s="";process.stdin.on("data",d=>s+=d).on("end",()=>console.log(JSON.parse(s).room_id))')
 URL_A=$(echo "$CREATED" | node -e 'let s="";process.stdin.on("data",d=>s+=d).on("end",()=>console.log(JSON.parse(s).invite_urls[0]))')
 URL_B=$(echo "$CREATED" | node -e 'let s="";process.stdin.on("data",d=>s+=d).on("end",()=>console.log(JSON.parse(s).invite_urls[1]))')
+OBSERVER=$(echo "$CREATED" | node -e 'let s="";process.stdin.on("data",d=>s+=d).on("end",()=>console.log(JSON.parse(s).observer_url))')
 TOKEN_A="${URL_A##*/}"
 TOKEN_B="${URL_B##*/}"
 PID_A=$(echo "$CREATED" | node -e 'let s="";process.stdin.on("data",d=>s+=d).on("end",()=>console.log(JSON.parse(s).participants[0].id))')
 
 echo "Room: $ROOM"
+echo "Observer URL for the human (read-only, whose turn it is): $OBSERVER"
 echo "Invite URL for Agent B (the human forwards this ONCE): $URL_B"
 
 echo