[ENHANCEMENT]: Aggregate subtask costs and improve hierarchy visualization in Orchestrator/Boomerang mode

[hannesrudolph]

Problem Statement

Currently, the Orchestrator mode lacks clear task hierarchy visualization, making it difficult for users (especially new ones) to understand the relationship between parent tasks and their subtasks. Additionally, it's hard to keep track of the total money spent across an orchestrated workflow since only the current active task's cost is visible.

Feature Description

This feature would improve the orchestrator experience by:

1. Aggregating costs from all subtasks into the parent task display
2. Providing clear visual hierarchy to show task relationships throughout the interface
3. Grouping subtasks with their parent tasks in history and UI components

The cost aggregation would be displayed in a collapsible/expandable format in the TaskHeader, showing a total by default with detailed breakdown available on click.

Use Cases

• Budgeting and cost control during development workflows
• Post-task analysis and reporting of orchestrated workflows
• Real-time decision making about continuing with expensive subtasks
• Improved new user experience by making orchestrator mode more intuitive and accessible

Acceptance Criteria

• TaskHeader displays aggregated cost for top-level Orchestrator tasks
• Cost display is collapsible/expandable: shows total by default, detailed breakdown on click
• Detailed breakdown shows hierarchical view with nested subtasks and proper indentation
• Real-time cost updates as subtasks complete
• Cost data persists across VSCode restarts through task history/checkpoints
• Subtasks are grouped with parent tasks in history and UI
• Clear visual hierarchy shows parent-child task relationships throughout the interface
• Only top-level Orchestrator tasks show aggregated costs (keep scope focused)
• Existing cost calculation and display functionality remains unchanged for non-orchestrator tasks

Technical Requirements

Display Specifications

• Location: TaskHeader component
• Format: Collapsible/expandable display - total by default, breakdown on click
• Update Frequency: Real-time updates as subtasks complete
• Breakdown Detail: Hierarchical view with nested subtasks and indentation showing parent-child relationships

Implementation Scope

• Scope: Only for top-level Orchestrator tasks - keep it simple and focused
• Persistence: Persist across VSCode restarts - save cost data to task history/checkpoints
• Task Grouping: Group subtasks with parent tasks in history and throughout the interface
• Cost Aggregation: Sum costs from parent task and all completed subtasks
• UI Hierarchy: Clear visual indication of task hierarchy and parent-child relationships

Additional Notes

The user emphasized that task hierarchy visualization is more important than cost aggregation. The current orchestrator experience has subtasks appearing as separate, disconnected items in history with no clear parent relationship. This makes it hard to understand which subtasks belong to which orchestrator task, and there's no visual indication that you're working within an orchestrated workflow. Improving this would make the orchestrator process far more accessible for new users. The issue that describes this disconnect is #8081