fix: resolve broken forked sessions with compactions due to missing parent-child message references

[ryanwyler]

Summary

Forking a compacted session fails with:

prompt is too long: 203573 tokens > 200000 maximum

The bug was introduced in commit 9a0735de7 on October 6, 2025 with the commit message "Add session forking functionality and simplify remove logic".

This was a bug from day one of the fork feature - it just wasn't noticed until compaction logic (which relies on parent-child relationships) was added later.

The Bug

When forking was originally implemented, each message gets a new id via Identifier.ascending("message"), but the code never updated the parentID field. Assistant messages have a parentID that points to their corresponding user message. After forking, those parentID values still reference the old message IDs from the source session, which don't exist in the forked session.

Fix

Maintain an ID mapping (old -> new) when cloning messages, updating parentID references to point to the new IDs. This restores the parent-child relationships that filterCompacted() relies on to detect compaction breakpoints.