sessionList: don't treat unrecognized text as session name

resolveFullSID returned the input text as a fallback when no session
matched, causing acme built-in commands like Del to be intercepted
and passed to openChat instead of back to acme via WriteEvent.

main.go

@@ -97,7 +97,7 @@ func (sl *sessionList) resolveFullSID(text string) string {
 			return sid
 		}
 	}
-	return text
+	return ""
 }
 
 func (sl *sessionList) eventLoop() {