Feature request: Expose autocompact settings in statusline JSON input

Summary

Custom statusline scripts receive context window info (current_tokens, context_size), but there's no way to know whether autocompact is enabled or what threshold triggers it.

When autocompact is enabled (the default), compaction triggers before 100% capacity. A progress bar showing "77%" might actually be at the effective limit, but the script can't know this.

Proposed Solution

Add two fields to the statusline JSON:

{
  "context_window": {
    "autocompact_enabled": true,
    "autocompact_threshold": 0.775
  }
}

This would let scripts calculate the effective max and show accurate "X% until compaction" displays.

Why This Matters

Users see the bar repeatedly fill to ~77% then drop, which feels confusing. With these fields, statuslines could show progress relative to the actual compaction point.

Thanks for considering this!