Feature: configurable auto-compaction threshold

Feature request

Allow users to configure the context window percentage at which auto-compaction triggers, via a setting in settings.json (user or project level).

{
  "autoCompactionThreshold": 80
}

If not specified, the current default behavior applies.

Motivation

With the statusLine feature, users can now monitor context window usage in real time. However, there's no way to control when auto-compaction fires. In my experience, compaction triggered at ~76% used (24% remaining) — well before the context window was exhausted.

For long-running sessions with heavy context (architecture decisions, multi-file reviews), earlier compaction discards useful context prematurely. For other workflows, later compaction might risk losing important state.

A configurable threshold would let users tune the trade-off between context preservation and safety margin based on their workflow.

Proposed behavior

• New optional setting: autoCompactionThreshold (integer, 1–99, percentage of context used)
• When context usage reaches the threshold, auto-compaction fires as it does today
• If omitted, current default behavior is unchanged
• Could be set at user level (~/.claude/settings.json) or project level (.claude/settings.json)