Get Task Graph

Return the task dependency graph as JSON (nodes + edges + critical path).

Builds a DAG from all current tasks and returns:

• nodes: list of {id, role, status, estimated_minutes, title}
• edges: list of {from, to, type, semantic_type}
• critical_path: ordered list of task IDs on the longest chain
• critical_path_minutes: total estimated minutes on the critical path
• parallel_width: max tasks that can run concurrently
• bottlenecks: task IDs that block the most downstream work