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/memory/memory.go

@@ -1281,6 +1281,19 @@ func (m *MemoryStorage) GetBlockedIssues(ctx context.Context, filter types.WorkF
 	return results, nil
 }
 
+// IsBlocked checks if an issue is blocked by open dependencies (GH#962).
+// Returns true if the issue has open blockers, along with the list of blocker IDs.
+func (m *MemoryStorage) IsBlocked(ctx context.Context, issueID string) (bool, []string, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	blockers := m.getOpenBlockers(issueID)
+	if len(blockers) == 0 {
+		return false, nil, nil
+	}
+	return true, blockers, nil
+}
+
 // getAllDescendants returns all descendant IDs of a parent issue recursively
 func (m *MemoryStorage) getAllDescendants(parentID string) map[string]bool {
 	descendants := make(map[string]bool)