Add dependency and dependent counts to bd list JSON output (#198)

When using `bd list --json`, each issue now includes:
- `dependency_count`: Number of issues this issue depends on
- `dependent_count`: Number of issues that depend on this issue

This provides quick access to dependency relationship counts without
needing to fetch full dependency lists or run multiple bd show commands.

Performance:
- Uses single bulk query (GetDependencyCounts) instead of N individual queries
- Overhead: ~26% for 500 issues (24ms vs 19ms baseline)
- Avoids N+1 query problem that would have caused 2.2x slowdown

Implementation:
- Added GetDependencyCounts() to Storage interface for bulk counting
- Efficient SQLite query using UNION ALL + GROUP BY
- Memory storage implementation for testing
- Moved IssueWithCounts to types package to avoid duplication
- Both RPC and direct modes use optimized bulk query

Tests:
- Added comprehensive tests for GetDependencyCounts
- Tests cover: normal operation, empty list, nonexistent IDs
- All existing tests continue to pass

Backwards compatible: JSON structure is additive, all original fields preserved.

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

Co-authored-by: Claude <noreply@anthropic.com>

internal/rpc/server_issues_epics.go

@@ -313,7 +313,28 @@ func (s *Server) handleList(req *Request) Response {
 		issue.Labels = labels
 	}
 
-	data, _ := json.Marshal(issues)
+	// Get dependency counts in bulk (single query instead of N queries)
+	issueIDs := make([]string, len(issues))
+	for i, issue := range issues {
+		issueIDs[i] = issue.ID
+	}
+	depCounts, _ := store.GetDependencyCounts(ctx, issueIDs)
+
+	// Build response with counts
+	issuesWithCounts := make([]*types.IssueWithCounts, len(issues))
+	for i, issue := range issues {
+		counts := depCounts[issue.ID]
+		if counts == nil {
+			counts = &types.DependencyCounts{DependencyCount: 0, DependentCount: 0}
+		}
+		issuesWithCounts[i] = &types.IssueWithCounts{
+			Issue:           issue,
+			DependencyCount: counts.DependencyCount,
+			DependentCount:  counts.DependentCount,
+		}
+	}
+
+	data, _ := json.Marshal(issuesWithCounts)
 	return Response{
 		Success: true,
 		Data:    data,