docs(search): don't threshold JSON score as Tavily

S1: skill recipe now says result score is merged rank. Also send
non-zero --min-score to the server (N1), sort include_domains (N2),
and note that search_queries counts each sub-query (N3).

cmd/sift/searchcmd.go

@@ -96,7 +96,7 @@ func searchMain() {
 	if *exact {
 		body["exact_match"] = true
 	}
-	if *minScore > 0 {
+	if *minScore != 0 {
 		body["min_score"] = *minScore
 	}
 	base, key, err := loadClient()