PreCompact hook: add blocking mode for interactive turn before compaction

Problem

When context compaction triggers (manual /compact or auto), there's no way to run a skill or give the model an interactive turn before compaction begins. The PreCompact hook fires during compaction and its stdout becomes a system-reminder in the post-compaction context — but by then, conversation history is already lost.

This means workflows like /handoff (saving session state before context is discarded) can't be triggered automatically before compaction.

Proposal

Add a blocking mode to PreCompact hooks that:

1. Pauses compaction before summarization begins
2. Gives the model an interactive turn with the hook's stdout as context
3. Lets the model run skills (e.g., /handoff to save session state) or ask the user a question
4. Resumes compaction after the interactive turn completes (or user approves)

Example config

{
  "PreCompact": [
    {
      "hooks": [
        {
          "type": "command",
          "command": "~/.claude/hooks/pre-compact-handoff.sh",
          "blocking": true
        }
      ]
    }
  ]
}

When blocking: true, the hook's stdout triggers a model turn before compaction proceeds. The model can then invoke skills, interact with the user, or perform cleanup — then compaction continues.

Alternative: lifecycle-event-to-skill binding

A more general solution would be native event-to-skill triggers:

{
  "lifecycle": {
    "preCompact": ["/handoff"]
  }
}

This would allow skills to run automatically on lifecycle events without needing shell command intermediaries.

Use case

Session handoff (/handoff) generates a .handoff.md file capturing goal, decisions, changed files, and next steps — so a fresh session can resume with full context in ~2K tokens. Running this automatically before compaction would prevent context loss without requiring the user to remember to run it manually.