Fix multiple issues and renumber database

- bd-170: Implement hybrid sorting for ready work (recent 48h first, then oldest)
- bd-87: Use safer null-byte placeholders in ID remapping
- bd-92: Make auto-flush debounce configurable via BEADS_FLUSH_DEBOUNCE
- bd-171: Fix nil pointer dereference in renumber command
- Delete spurious test issues (bd-7, bd-130-134)
- Renumber database from 171 down to 144 issues

internal/storage/sqlite/collision.go

@@ -429,7 +429,7 @@ func buildReplacementCache(idMapping map[string]string) ([]*idReplacementCache,
 		cache = append(cache, &idReplacementCache{
 			oldID:       oldID,
 			newID:       newID,
-			placeholder: fmt.Sprintf("__PLACEHOLDER_%d__", i),
+			placeholder: fmt.Sprintf("\x00REMAP\x00_%d_\x00", i),
 			regex:       re,
 		})
 		i++