[Feature] Proactive /compact at configurable context threshold to prevent hard "context full" crashes

Summary

When running long, tool-heavy sessions (bulk file reads, multiple subagents), context can fill to 100% faster than auto-compact can reclaim space. At that point, /compact becomes unresponsive and the user is forced to manually restart the session, losing in-flight work.

Request: Allow configuring a proactive compact threshold (e.g., 15% remaining) that triggers a full /compact cycle before the context becomes irrecoverable.

Use Case

Reading 151 template files across a codebase audit — each file read consumes context, subagent outputs return large results, and the session hit "context full" twice. The only recovery was manually copying the /compact summary into a new session.

Key pain points:

• No warning: Context went from usable to "full" with no actionable alert
• Auto-compact insufficient: Built-in compaction couldn't keep up with the volume of tool results
• /compact failed at 100%: Once full, /compact itself couldn't execute
• Subagent outputs are large: Background agents return full results that spike context usage

Proposed Behavior

// In .claude/settings.json
{
  "compactThreshold": 0.15,
  "compactWarningThreshold": 0.25
}

1. At compactWarningThreshold → show persistent banner: "Context at 75% — consider running /compact"
2. At compactThreshold → auto-run /compact (or prompt user to confirm)
3. Never let context reach the irrecoverable 100% state

Workarounds Currently Used

• Writing findings to persistent memory files as work progresses (survives session restarts)
• Manually running /compact after each major milestone
• Breaking large tasks into smaller sessions

Related Issues

• #5720 — Warning before auto-compact (similar direction, but no configurable threshold)
• #16209 — Context full with subagents, /compact unresponsive (describes the exact failure mode)
• #42 — Original auto-compact request

Environment

• Claude Code on macOS (Darwin 24.6.0)
• Model: claude-opus-4-6
• Session type: Heavy tool use (150+ file reads, 6 background agents, persistent memory writes)