[Feature Request] Statusline JSON missing aggregate token counts across subagents

Bug Description
Title: Statusline JSON should include aggregate token counts across subagents

Description:

The statusline JSON data provides context_window.total_input_tokens and context_window.total_output_tokens, but these only count the main conversation's tokens — subagent token usage is excluded.

For example, in a session where 3 subagents used ~161k tokens combined, the statusline reported only 9.4k total tokens. Meanwhile, cost.total_cost_usd correctly aggregates across subagents ($0.62).

Request: Add aggregate token fields (e.g., cost.total_input_tokens and cost.total_output_tokens) that include subagent usage, similar to how cost.total_cost_usd already aggregates costs across the full session.

This would allow custom statuslines to display accurate total token consumption.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.71
• Feedback ID: 0e7c549f-c757-42f1-8f33-b6171439f420