Feature request: expose context window usage to hooks and support threshold-based triggers

Summary

Enable hooks (or a new hook event) to access context window usage metrics and trigger actions at configurable thresholds — for example, automatically saving progress and clearing the context window at 50% usage.

Motivation

Long-running sessions risk losing valuable context when automatic compression kicks in unexpectedly. Users currently have no programmatic way to:

1. Detect how full the context window is from a hook or skill
2. Trigger memory-saving actions based on a threshold
3. Automate graceful context management

This forces manual monitoring via /context and manual invocation of /clear.

Proposed Changes

Any combination of:

• Expose context_usage_percent (or token counts) in hook input data — even just adding it to UserPromptSubmit or PostToolUse input would unlock threshold-based logic in user scripts
• Add a ContextThreshold hook event — fires when context usage crosses a configurable percentage (e.g., 50%, 75%)

Use Cases

• Auto-save session progress to memory files before context gets compressed
• Trigger /clear at a controlled point rather than hitting the limit unexpectedly
• Alert the user (via hook stderr) when context is getting large
• Delegate remaining work to subagents when context is nearly full