fix: prevent closing issues with open blockers (GH#962)

Added IsBlocked method to Storage interface that checks if an issue is
in the blocked_issues_cache and returns the blocking issue IDs.

The close command now checks for blockers before allowing an issue to
be closed:
- If an issue has open blockers, closing is blocked with an error message
- The --force flag overrides this check
- Works in both daemon mode (RPC) and direct storage mode
- Also handles cross-rig routed IDs

This addresses the bug where agents could close a bead even when it
depends on an open bug/issue.

Closes #962

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/storage_test.go

@@ -98,6 +98,9 @@ func (m *mockStorage) GetReadyWork(ctx context.Context, filter types.WorkFilter)
 func (m *mockStorage) GetBlockedIssues(ctx context.Context, filter types.WorkFilter) ([]*types.BlockedIssue, error) {
 	return nil, nil
 }
+func (m *mockStorage) IsBlocked(ctx context.Context, issueID string) (bool, []string, error) {
+	return false, nil, nil
+}
 func (m *mockStorage) GetEpicsEligibleForClosure(ctx context.Context) ([]*types.EpicStatus, error) {
 	return nil, nil
 }