When a long-running subagent finishes (success/fail/abort), the main agent should:
- Notify the user in the interactive session when idle (push-style, no polling), and
- Persist a concise, searchable record even if the interactive session is disconnected/terminated.
Status (PR #323): PR #323 introduces pi.events so hooks can notify the user while the agent is idle. It also mentions PI_ASYNC_RESULT for writing an async result artifact. This proposal adds a durable session-history record by appending subagent lifecycle events to the existing session .jsonl via SessionManager.
PR #323 already enables (1) via pi.events + hooks. This adds (2) using the existing SessionManager JSONL session file.