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/conversation_loop.py

@@ -4561,6 +4561,7 @@ def run_conversation(
         original_user_message=original_user_message,
         final_response=final_response,
         interrupted=interrupted,
+        messages=messages,
     )
 
     # Background memory/skill review — runs AFTER the response is delivered