"Summarize before here" option in rewind menu

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The current "Summarize from here" in the rewind menu compresses the selected message and everything after it, keeping earlier messages intact. This is great when you want to preserve your original instructions and compress verbose middle/end sections.

However, the inverse would be equally useful — compressing everything before a selected point while keeping recent messages fully intact. In long sessions, the early back-and-forth (debugging, exploration, planning) often becomes less relevant, while the recent context (current task, recent decisions, latest code changes) is what matters most.

Proposed Solution

Add a "Summarize before here" option to the rewind menu alongside the existing options:

1. Restore code and conversation
2. Restore conversation
3. Restore code
4. Summarize from here
5. Summarize before here (new)

Behavior:

• All messages before the selected checkpoint get replaced with a compact AI-generated summary
• The selected message and everything after it stays fully intact
• Same as "Summarize from here" but in reverse — no files on disk are changed, original messages preserved in the session transcript
• Optionally accepts focus instructions (same as /compact) to guide what the summary retains

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

Long sessions where early exploration/debugging consumed significant context, but the user wants to keep their recent work (last few turns) in full detail. Today the only option is /compact which compresses everything, or "Summarize from here" which compresses the wrong direction for this scenario.

Additional Context

_No response_