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/engine_test.go

@@ -14,12 +14,13 @@ type fakeSrc struct {
 	name    string
 	hits    []Hit
 	warn    string
+	notes   []string
 	delay   time.Duration
 	credits int
 }
 
 func (f fakeSrc) Name() string { return f.name }
-func (f fakeSrc) Search(ctx context.Context, query string, k int) SourceOut {
+func (f fakeSrc) Search(ctx context.Context, query string, k int, _ Filter) SourceOut {
 	if f.delay > 0 {
 		select {
 		case <-time.After(f.delay):
@@ -27,7 +28,7 @@ func (f fakeSrc) Search(ctx context.Context, query string, k int) SourceOut {
 			return SourceOut{Warn: f.name + ": timeout"}
 		}
 	}
-	return SourceOut{Hits: f.hits, Warn: f.warn, Credits: f.credits}
+	return SourceOut{Hits: f.hits, Warn: f.warn, Credits: f.credits, Notes: f.notes}
 }
 
 type stubEmb struct {