SessionEnd hook: add 'prompt' type for inline AI processing

Problem

SessionEnd hooks can only run shell commands. Any AI processing of session context requires hacking together a claude --resume subprocess, which introduces:

1. Recursion — resumed session ends → triggers SessionEnd again → infinite loop
2. Nested Claude bug — --resume --print > file silently fails when called from within Claude (stdout doesn't redirect)
3. Exit blocking — hook blocks session exit unless you fully detach with nohup ... </dev/null >/dev/null 2>&1 &
4. Workaround overhead — env var recursion guards, --no-session-persistence, FD management

All of this just to do something simple: process the session context with a model after the session ends.

Proposed solution

Add a prompt hook type:

{
  "hooks": {
    "SessionEnd": [
      {
        "matcher": "clear|prompt_input_exit",
        "hooks": [
          {
            "type": "prompt",
            "model": "haiku",
            "prompt": "Evaluate this session and generate a note if worth documenting. Write to ~/notes/sessions/ using the Write tool."
          }
        ]
      }
    ]
  }
}

The hook runtime already has session_id, transcript_path, and full session context. A prompt type would:

• Pass session context to the specified model directly (no --resume needed)
• Run as a hook-scoped invocation (no SessionEnd re-trigger, no recursion)
• Handle async execution internally (no exit blocking)
• Allow tool use (Read/Write) for the model to act on its evaluation

Current workaround

~80 lines of shell script that:

• Parses JSON input for session_id/reason
• Filters by exit reason
• Guards against recursion via env var
• Spawns claude --resume $SESSION_ID --no-session-persistence --dangerously-skip-permissions via nohup with all FDs closed
• Hopes it works

Why this matters

Session-end AI processing is a natural use case (auto-summarization, memory extraction, cleanup). The current command type forces users through a fragile gauntlet of subprocess management that shouldn't be necessary.

🤖 Generated with Claude Code