Configurable threshold for when 'X% until auto-compact' indicator appears

Summary

The built-in "X% until auto-compact" footer indicator only appears close to the auto-compact threshold. There is no way to configure when it becomes visible.

What I'd like

A setting to control the visibility threshold, e.g.:

"autoCompactIndicatorThreshold": 35

This would show the "X% until auto-compact" indicator when remaining context drops to 35% or below (i.e., 65%+ used). The default could remain as-is for users who don't set it.

Why

During long sessions with heavy tool use (file reads, MCP calls, image analysis), it's useful to know earlier that context is running low — not just in the final moments before compaction. A 35% threshold gives enough warning to wrap up a logical unit of work, commit, or start a fresh session.

Current workaround

The statusLine feature can approximate this by reading context_window.used_percentage from stdin JSON and conditionally displaying a message. However, this occupies the single statusLine slot and doesn't integrate with the native indicator styling.

Related issues

• #43831 — Suppress the indicator (opposite direction)
• #38971 — Show context % in UI (broader; marked duplicate)
• #40018 — Expose context usage metrics
• #40757, #43981, #34925 — Configurable auto-compact threshold (the behavior, not the indicator visibility)

This request is narrower: keep the existing indicator and auto-compact behavior, just let users configure when the indicator becomes visible.