Adds an optional `messages` keyword to the `MemoryProvider.sync_turn`
contract so external/community memory plugins can receive the OpenAI-style
conversation message list for the completed turn — including assistant tool
calls and tool result content — not just the final assistant text.
Dispatch uses signature inspection (`_provider_sync_accepts_messages`): only
providers that declare a `messages` parameter (or `**kwargs`) receive it; all
existing in-tree providers keep their legacy text-only signature and are
called unchanged. No structured-trace envelope is added to core — providers
reconstruct whatever they need from the standard message list.
Also documents Memori as a standalone community memory provider.
Salvaged from #28065 — rebased onto current main.
Co-authored-by: Dave Heritage <david@memorilabs.ai>
