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.go

@@ -110,6 +110,7 @@ type Storage interface {
 	// Ready Work & Blocking
 	GetReadyWork(ctx context.Context, filter types.WorkFilter) ([]*types.Issue, error)
 	GetBlockedIssues(ctx context.Context, filter types.WorkFilter) ([]*types.BlockedIssue, error)
+	IsBlocked(ctx context.Context, issueID string) (bool, []string, error) // GH#962: Check if issue has open blockers
 	GetEpicsEligibleForClosure(ctx context.Context) ([]*types.EpicStatus, error)
 	GetStaleIssues(ctx context.Context, filter types.StaleFilter) ([]*types.Issue, error)
 	GetNewlyUnblockedByClose(ctx context.Context, closedIssueID string) ([]*types.Issue, error) // GH#679