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/dcache/dcache.go

@@ -11,8 +11,6 @@ import (
 	"log/slog"
 	"math"
 	"os"
-	"sort"
-	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -181,17 +179,6 @@ func QuestionEmbedKey(question, embedModel, callerClass string) string {
 	return hex.EncodeToString(sum[:])
 }
 
-func SearchKey(query string, sources []string, k int, hour time.Time, callerClass string) string {
-	if callerClass == "" {
-		callerClass = ClassFleet
-	}
-	src := append([]string(nil), sources...)
-	sort.Strings(src)
-	bucket := hour.UTC().Format("2006-01-02T15")
-	sum := sha256.Sum256([]byte(NormalizeQuestion(query) + "\x00" + strings.Join(src, ",") + "\x00" + strconv.Itoa(k) + "\x00" + bucket + "\x00" + callerClass))
-	return hex.EncodeToString(sum[:])
-}
-
 type DigestHit struct {
 	Text       string
 	ChunksUsed int