fix(core): preserve compaction after revert

[opencode-agent]

Summary

• commit a staged session revert before admitting manual compaction
• prevent the next prompt from truncating a completed compaction at the old revert boundary
• cover the ordering with a core regression test

Reproduction

1. Revert the latest message.
2. Compact the session while the revert is staged.
3. Send a follow-up prompt.
4. The revert commit removes the compaction message, so the follow-up does not use its summary.

Verification

• bun test test/session-compact.test.ts --timeout 30000
• bun typecheck
• OpenCode Drive /undo → /compact → follow-up flow; verified the follow-up model request contains the compaction summary

Requested by: @rekram1-node (Aiden Cline via Slack)