Compact image-heavy history before reuse

[cbusillo]

Summary

• add reusable history compaction for old images, image-generation payloads, and large tool outputs
• preserve the active turn's newest recorded item so fresh images still reach the model
• compact response items and replacement histories before rollout storage so new session files do not keep raw image ballast

Closes #45

Validation

• cargo test -p code-core history_compaction --no-default-features
• ./build-fast.sh