Feature request: 'Summarize before here' to complement 'Summarize from here'

Feature Request

Current behavior: The conversation context menu offers "Summarize from here" which summarizes all messages from the chosen message onwards.

Requested behavior: Add a "Summarize before here" option that summarizes all conversation prior to the chosen message, while keeping the messages from that point onwards in full detail.

Use case

In long implementation sessions, the early conversation (planning, exploration, initial reviews) becomes less important than the recent work (active code changes, review feedback, CI fixes). Currently, "Summarize from here" compresses the tail, but often it's the head of the conversation that's stale and should be compressed while keeping the recent context fine-grained.

This would let users preserve full detail on the work they're actively iterating on while freeing up context window for the older planning/exploration messages that are no longer load-bearing.

Example

A 200-message session where messages 1-150 were planning and initial implementation, and 151-200 are active review feedback iteration. Right-clicking message 151 and choosing "Summarize before here" would compress 1-150 into a summary while keeping 151-200 intact.