[FEATURE][vscode] Track token usage from sub-agent calls in Copilot agent mode

[tianzheng-zhou]

Feature Request

When using GitHub Copilot's agent mode in VS Code, the extension does not appear to track token usage from sub-agent calls. Only the main agent's token consumption is recorded, while tokens consumed by sub-agents are missing from the statistics.

Expected Behavior

All token usage should be captured, including tokens consumed by sub-agents invoked during an agent mode session.

Current Behavior

Sub-agent token usage is not reflected in the usage statistics, leading to underreported total token consumption.

[github-actions]

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