feat: add daily challenge seed generation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

game/daily.go

@@ -0,0 +1,11 @@
+package game
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+)
+
+func DailySeed(date string) int64 {
+	h := sha256.Sum256([]byte("catacombs:" + date))
+	return int64(binary.BigEndian.Uint64(h[:8]))
+}