feat(analysis/bottlenecks): implement bottlenecks function using graphology-metrics betweenness centrality
- Implements bottlenecks(graph: TaskGraph): BottleneckResult[] with normalized scores (0.0-1.0)
- Uses graphology-metrics centrality.betweenness with normalized: true
- Returns array sorted by score descending, includes all nodes (even score 0)
- Handles empty graph edge case (returns empty array)
- 20 unit tests covering: linear chain, star graph, independent nodes,
disconnected graph, diamond, empty graph, single node