store: voiding a fact drops its memory vectors (V-470)

Revert voided the fact row and left the vector, so recall kept serving the
voided fact's utterance and the documented repair reported success on a box that
stayed broken. There was no way to repair a poisoned box at all.

DeletePrefix covers every vector for the key, earlier rows included: their values
are superseded, and a superseded value has no business claiming a turn. It is
best-effort — the audit trail is already committed, and a fact that is voided but
still recallable beats a void that failed.

internal/store/memory.go

@@ -148,6 +148,27 @@ func (m *MemoryStore) Delete(ctx context.Context, id string) error {
 	return nil
 }
 
+// DeletePrefix removes every vector whose id starts with prefix and returns
+// how many went. Same escaping as ByPrefix, so a key containing % or _ cannot
+// widen the delete.
+//
+// It exists for the repair half of a revert (#470). Voiding a fact row left
+// its vector in the index, so recall kept serving the voided fact's utterance
+// and the documented repair did not repair.
+func (m *MemoryStore) DeletePrefix(ctx context.Context, prefix string) (int64, error) {
+	pattern := escapeLike(prefix) + "%"
+	res, err := m.db.ExecContext(ctx,
+		`DELETE FROM memory_vectors WHERE id LIKE ? ESCAPE '\'`, pattern)
+	if err != nil {
+		return 0, fmt.Errorf("memory: delete prefix %q: %w", prefix, err)
+	}
+	n, err := res.RowsAffected()
+	if err != nil {
+		return 0, fmt.Errorf("memory: delete prefix %q: rows affected: %w", prefix, err)
+	}
+	return n, nil
+}
+
 // escapeLike neutralises the LIKE wildcards in a literal prefix.
 func escapeLike(s string) string {
 	r := strings.NewReplacer(`\`, `\\`, `%`, `\%`, `_`, `\_`)