v0.49.34

This patch stops OpenCode 1 sessions from ending up with two summary markers after a failed save.

Upgrading

A failed save could leave a second summary marker on OpenCode 1: when Magic Context moves the point OpenCode loads history from, it writes a marker into OpenCode's database and then records that marker in its own database. If the second write failed, the next history update didn't know the first marker was its own and added another. Magic Context now records the marker it intends to write before writing it, and the next update adopts a marker that's already in place instead of adding one. Cases it can't settle with certainty are logged and left untouched.
- Cleanup of leftover markers now runs only on a pass that is already rebuilding the cached prompt, so it can never change what a cached request loads.
- OpenCode 2 and Pi: unchanged. They don't use this marker.