[FEATURE]: V2 plugin support for managing context-window transitions

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

The related V1 proposal is linked below; I did not find an existing request for this V2 capability.

Describe the enhancement you want to request

I’m building a V2 plugin that uses Astra’s native history and notes to continue across context windows without a generated summary.

The plugin can control which messages are sent to the model, but OpenCode’s builtin automatic compaction can run before that request hook or during context-overflow recovery. There doesn’t appear to be a supported way for a plugin to take over context-window transitions for the sessions it manages.

Could V2 expose a way to support this? The motivation is to integrate model-native context management through a plugin without requiring a custom OpenCode build.

I reproduced both compaction paths in an isolated runtime fixture on 0.0.0-beta-19157.

Related: #27366 proposed a compaction bypass for V1 but was closed for inactivity.