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/types/types.go

@@ -147,6 +147,19 @@ type Dependency struct {
 	CreatedBy   string         `json:"created_by"`
 }
 
+// DependencyCounts holds counts for dependencies and dependents
+type DependencyCounts struct {
+	DependencyCount int `json:"dependency_count"` // Number of issues this issue depends on
+	DependentCount  int `json:"dependent_count"`  // Number of issues that depend on this issue
+}
+
+// IssueWithCounts extends Issue with dependency relationship counts
+type IssueWithCounts struct {
+	*Issue
+	DependencyCount int `json:"dependency_count"`
+	DependentCount  int `json:"dependent_count"`
+}
+
 // DependencyType categorizes the relationship
 type DependencyType string