feat: show root orchestration cost tally for subtasks

[roomote-v0]

Related GitHub Issue

Closes: #11928

Description

This PR attempts to address Issue #11928 by displaying the full orchestration cost (aggregated across all tasks) when viewing a subtask spawned by an orchestrator.

How it works:

• ChatView: When the current task has a rootTaskId (i.e., it is a subtask), we request aggregated costs for the root task via the existing getTaskWithAggregatedCosts message. The response is stored in the existing aggregatedCostsMap.
• TaskHeader: A new optional rootOrchestrationCost prop is passed from ChatView. When present and > 0, it renders the orchestration total cost in both the collapsed and expanded views of the task header.
• i18n: Two new English translation keys added: chat:costs.orchestrationLabel and chat:costs.orchestrationTotal.

No backend changes were needed -- this leverages the existing rootTaskId on HistoryItem and the existing aggregateTaskCostsRecursive infrastructure.

Test Procedure

• Added 4 new tests to TaskHeader.spec.tsx covering orchestration cost display (present, absent, zero value).
• All 25 TaskHeader tests pass. All 12 aggregateTaskCosts tests pass.
• Full lint and type-check pass across the monorepo.

Manual testing steps:

1. Start an orchestrator task that spawns subtasks.
2. Navigate to a subtask view.
3. Verify the orchestration total cost appears next to the task's own cost in both collapsed and expanded header views.

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue.
• Scope: Changes are focused on the linked issue.
• Self-Review: Performed a thorough self-review.
• Testing: New tests added to cover changes.
• Documentation Impact: No documentation updates required.
• Contribution Guidelines: Read and agree to Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

Feedback and guidance are welcome.

Interactively review PR in Roo Code Cloud