[Feature Request] Expose main chat token usage separately in statusLine JSON

Currently, the statusLine JSON only provides total_cost_usd which aggregates costs across the main conversation and all
subagents. There's no way to see the token usage for just the main chat context.

Requested addition to statusLine JSON:

"cost": {
"total_cost_usd": 0.204,
"main_chat_input_tokens": 45000,
"main_chat_output_tokens": 12000,
"context_window": 200000,
...
}

Use case: I want to display a color-coded progress bar in my statusLine showing how much of the context window is consumed by
the main conversation. This helps me know when to start a new chat or use /compact. The current total_cost_usd includes subagent
costs which don't affect the main chat's context window.

Why this matters: Subagents run in separate context windows, so their token usage doesn't consume the main chat's 200K limit.
Showing aggregated tokens misrepresents how close I am to hitting the context limit.

Environment Info

• Platform: darwin
• Terminal: tmux
• Version: 2.0.56
• Feedback ID: