feat(search): P3.1 filters and multi-query merge

Native include/exclude domains, time_range, topic, exact_match, min_score,
and up to 4 queries fused with RRF. Honesty notes stay off the Warn/fail
path. Tavily keeps include_answer false and sets auto_parameters false.
Cache key is canonical JSON. CLI flags and skill recipe included.

internal/search/rank.go

@@ -105,6 +105,7 @@ func rrfMerge(lists [][]Hit) []Hit {
 			sc := 1.0 / float64(rrfK+r+1)
 			if a, ok := seen[c]; ok {
 				a.s += sc
+				keepDate := firstDate(a.h.PublishedDate, h.PublishedDate)
 				if len(h.Snippet) > len(a.h.Snippet) {
 					src := a.h.Source
 					keep := a.h.URL
@@ -112,6 +113,7 @@ func rrfMerge(lists [][]Hit) []Hit {
 					a.h.Source = src
 					a.h.URL = keep
 				}
+				a.h.PublishedDate = keepDate
 				continue
 			}
 			cp := h