feat: expose completed-turn message context to memory providers

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>

agent/memory_provider.py

@@ -112,11 +112,22 @@ class MemoryProvider(ABC):
         that do background prefetching should override this.
         """
 
-    def sync_turn(self, user_content: str, assistant_content: str, *, session_id: str = "") -> None:
+    def sync_turn(
+        self,
+        user_content: str,
+        assistant_content: str,
+        *,
+        session_id: str = "",
+        messages: Optional[List[Dict[str, Any]]] = None,
+    ) -> None:
         """Persist a completed turn to the backend.
 
         Called after each turn. Should be non-blocking — queue for
         background processing if the backend has latency.
+
+        ``messages`` is the OpenAI-style conversation message list as of the
+        completed turn, including any assistant tool calls and tool results.
+        Providers that do not need raw turn context can ignore it.
         """
 
     @abstractmethod