feat(tombstone): add P2 code review improvements (bd-saa, bd-1bu, bd-nyt)

- Add partial index on deleted_at for efficient TTL queries
- Exclude tombstones from SearchIssues by default (new IncludeTombstones filter)
- Report tombstone count separately in GetStatistics
- Display tombstone count in bd stats output

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/storage/sqlite/queries.go

@@ -1186,6 +1186,10 @@ func (s *SQLiteStorage) SearchIssues(ctx context.Context, query string, filter t
 	if filter.Status != nil {
 		whereClauses = append(whereClauses, "status = ?")
 		args = append(args, *filter.Status)
+	} else if !filter.IncludeTombstones {
+		// Exclude tombstones by default unless explicitly filtering for them (bd-1bu)
+		whereClauses = append(whereClauses, "status != ?")
+		args = append(args, types.StatusTombstone)
 	}
 
 	if filter.Priority != nil {