Current session-memory hook only saves on /new command. Add option for periodic auto-save based on triggers.

Problem:
Memory is only saved when user explicitly runs /new, which means:

• Context is lost if session crashes
• Long sessions accumulate unsaved context
• Compaction events lose everything

Proposed solution:

Extend or create new hook: memory-autosave

Triggers:

• Token threshold (e.g., every 50k tokens used)
• Time interval (e.g., every 30 minutes)
• Pre-compaction (via Issue Add session:pre-compact event hook #11617)

Example config:

{
  "hooks": {
    "internal": {
      "entries": {
        "memory-autosave": {
          "enabled": true,
          "triggerTokens": 50000,
          "triggerMinutes": 30,
          "summarize": true,
          "maxMessagesPerSave": 20
        }
      }
    }
  }
}

Actions on trigger:

1. Extract last N messages
2. Optionally summarize with LLM (compression)
3. Append to memory/YYYY-MM/DD-{channel}.md
4. Keep cumulative daily log

Priority: 🟡 Medium