Maintainer scoping decision for the #51226 salvage: document that
select_context() is for engines that must REPLACE per-request context
(retrieval/routing) — pre_llm_call is inject-only by documented cache
design; that observation-only plugins should implement a MemoryProvider
(sync_turn) rather than a context engine, with on_turn_complete scoped
as the observation mirror for engines that already select; and that a
non-no-op select_context naturally changes the prompt-cache prefix on
turns where the selection changes — engines should return stable
selections when nothing changed.
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