fix(graph): pass subgraph to renderGraph to fix nil pointer crash

The renderGraph function was being called with nil instead of the
loaded subgraph, causing a nil pointer dereference in
computeDependencyCounts when iterating over subgraph.Dependencies.

Changes:
- Pass subgraph variable to renderGraph instead of nil
- Add defensive nil check in computeDependencyCounts
- Add test case for nil subgraph handling

Fixes GH#657

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/graph.go

@@ -425,6 +425,10 @@ func computeDependencyCounts(subgraph *TemplateSubgraph) (blocks map[string]int,
 	blocks = make(map[string]int)
 	blockedBy = make(map[string]int)
 
+	if subgraph == nil {
+		return blocks, blockedBy
+	}
+
 	for _, dep := range subgraph.Dependencies {
 		if dep.Type == types.DepBlocks {
 			// dep.DependsOnID blocks dep.IssueID