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>

run_agent.py

@@ -2302,6 +2302,7 @@ class AIAgent:
         original_user_message: Any,
         final_response: Any,
         interrupted: bool,
+        messages: list | None = None,
     ) -> None:
         """Mirror a completed turn into external memory providers.
 
@@ -2334,9 +2335,13 @@ class AIAgent:
         if not (self._memory_manager and final_response and original_user_message):
             return
         try:
+            sync_kwargs = {"session_id": self.session_id or ""}
+            if messages is not None:
+                sync_kwargs["messages"] = messages
             self._memory_manager.sync_all(
-                original_user_message, final_response,
-                session_id=self.session_id or "",
+                original_user_message,
+                final_response,
+                **sync_kwargs,
             )
             self._memory_manager.queue_prefetch_all(
                 original_user_message,