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/rpc/protocol.go

@@ -174,6 +174,7 @@ type CloseArgs struct {
 	Reason      string `json:"reason,omitempty"`
 	Session     string `json:"session,omitempty"`      // Claude Code session ID that closed this issue
 	SuggestNext bool   `json:"suggest_next,omitempty"` // Return newly unblocked issues (GH#679)
+	Force       bool   `json:"force,omitempty"`        // Force close even with open blockers (GH#962)
 }
 
 // CloseResult is returned when SuggestNext is true (GH#679)