[BUG] Statusline context_window tokens are cumulative and don't reset on /clear

Description

The statusline JSON API reports total_input_tokens and total_output_tokens as cumulative session totals, not current context usage. These values don't reset when running /clear.

Steps to Reproduce

1. Work in Claude Code until context builds up
2. Run /context - note actual usage
3. Check statusline JSON - note different (higher) values
4. Run /clear
5. Check statusline JSON again - values NOT reset

Expected Behavior

After /clear, statusline token counts should reset to reflect current context, not cumulative session usage.

Actual Behavior

Statusline shows cumulative tokens since Claude Code started:

• /context: 105k/200k (53%)
• Statusline JSON: 383k tokens (191%)

Evidence

"context_window": {
  "total_input_tokens": 325641,
  "total_output_tokens": 57717,
  "context_window_size": 200000
}

This was captured immediately after /clear.

Environment

• Version: 2.0.69
• OS: macOS

Related Issues

This is distinct from #13652 and #13776 which are about statusline not including system overhead tokens. This bug is specifically about cumulative values not resetting.