Feature Request: Auto-save session summary before compaction

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

When using /compact or when auto-compaction triggers, all/some conversation context is lost. Users who want to maintain a running log of their work across sessions have no way to automatically capture what happened before compaction.

Proposed Solution

Add a setting like "saveSnapshotOnCompact": true (or a path like "snapshotFile": "SNAPSHOT.md") that would:

1. Before compaction executes, have Claude automatically generate a session summary
2. Append that summary to a configured file (e.g., SNAPSHOT.md in project root)
3. Then proceed with compaction

Alternative Solutions

Why Hooks Don't Solve This - The current SessionStart hook with compact matcher fires an external shell script, but that script has no access to the conversation content. Only Claude can summarize what happened.

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

Developers working on long-term projects want a trail of all sessions from day 1, so future sessions (or other team members) can understand the history of changes and decisions made with Claude's help.

Additional Context

_No response_