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/storage/storage.go

@@ -25,6 +25,7 @@ type Storage interface {
 	GetDependents(ctx context.Context, issueID string) ([]*types.Issue, error)
 	GetDependencyRecords(ctx context.Context, issueID string) ([]*types.Dependency, error)
 	GetAllDependencyRecords(ctx context.Context) (map[string][]*types.Dependency, error)
+	GetDependencyCounts(ctx context.Context, issueIDs []string) (map[string]*types.DependencyCounts, error)
 	GetDependencyTree(ctx context.Context, issueID string, maxDepth int, showAllPaths bool, reverse bool) ([]*types.TreeNode, error)
 	DetectCycles(ctx context.Context) ([][]*types.Issue, error)