Feature Request: Hook events for session/context usage thresholds

Feature Request

Problem

There is no way to get notified when context window usage or session-level cumulative usage (e.g., Max plan token/cost budget) reaches certain thresholds. Users who are away or multitasking have no proactive warning before hitting limits.

These are two distinct concerns:

1. Context window — how full the current context is (triggers compaction when full)
2. Session usage — cumulative tokens/cost consumed across the entire conversation (subject to plan limits like Max plan)

Proposed Solution

Add two new hook events with configurable thresholds:

1. ContextUsageWarning — context window capacity

Fires when context window usage crosses a threshold (e.g., 70%, 80%, 90%).

{
  "ContextUsageWarning": [
    {
      "matcher": "70|80|90",
      "hooks": [
        {
          "type": "command",
          "command": "terminal-notifier -title 'Context Window Warning' -message 'Context window usage reached ${THRESHOLD}%' -sound default"
        }
      ]
    }
  ]
}

2. SessionUsageWarning — cumulative session usage / plan budget

Fires when session-level cumulative usage crosses a threshold relative to the plan limit.

{
  "SessionUsageWarning": [
    {
      "matcher": "70|80|90|100",
      "hooks": [
        {
          "type": "command",
          "command": "terminal-notifier -title 'Session Usage Warning' -message 'Session usage reached ${THRESHOLD}% of plan limit' -sound default"
        }
      ]
    }
  ]
}

Why

• PreCompact only fires once context is already full — too late to act
• /cost requires manual checking and is not automatable via hooks
• Users running long tasks (tests, multi-file refactors, subagents) benefit from early warnings to plan compaction, start a new session, or budget remaining usage
• This aligns well with the existing hooks system and would be straightforward to configure

Alternatives Considered

• Using PreCompact hook as a proxy for "context near 100%" — works but only covers one threshold and doesn't address session usage
• Polling /cost manually — not automatable via hooks