Feature Request: Hook event for context window threshold alerts

Summary

Add a new hook event that triggers when the context window reaches a configurable threshold (e.g., 50%, 75%, 90%).

Use Case

When working on long sessions, users may want to be notified before running out of context space. Currently, there's no way to automate this - users must manually run /context to check usage.

Proposed Solution

Add a new hook event like ContextWindowThreshold that:

• Triggers when context usage crosses a configurable percentage
• Provides token count and percentage in the hook's stdin JSON
• Allows users to run custom scripts (e.g., desktop notifications, sound alerts)

Example configuration:

{
  "hooks": {
    "ContextWindowThreshold": [
      {
        "matcher": "50",
        "hooks": [
          {
            "type": "command",
            "command": "notify-send 'Claude Code' 'Context at 50%'"
          }
        ]
      }
    ]
  }
}

Alternatives Considered

• Adding instructions in CLAUDE.md to remind Claude to warn the user (current workaround, but not automated)
• Using Stop hook to parse logs (not reliable, no API access to token metrics)

Additional Context

This would greatly improve UX for users who work on complex, long-running tasks.