fix: thread gateway user_id to memory plugins for per-user scoping (#5895)

Memory plugins (Mem0, Honcho) used static identifiers ('hermes-user',
config peerName) meaning all gateway users shared the same memory bucket.

Changes:
- AIAgent.__init__: add user_id parameter, store as self._user_id
- run_agent.py: include user_id in _init_kwargs passed to memory providers
- gateway/run.py: pass source.user_id to AIAgent in primary + background paths
- Mem0 plugin: prefer kwargs user_id over config default
- Honcho plugin: override cfg.peer_name with gateway user_id when present

CLI sessions (user_id=None) preserve existing defaults. Only gateway
sessions with a real platform user_id get per-user memory scoping.

Reported by plev333.

plugins/memory/mem0/__init__.py

@@ -202,7 +202,9 @@ class Mem0MemoryProvider(MemoryProvider):
     def initialize(self, session_id: str, **kwargs) -> None:
         self._config = _load_config()
         self._api_key = self._config.get("api_key", "")
-        self._user_id = self._config.get("user_id", "hermes-user")
+        # Prefer gateway-provided user_id for per-user memory scoping;
+        # fall back to config/env default for CLI (single-user) sessions.
+        self._user_id = kwargs.get("user_id") or self._config.get("user_id", "hermes-user")
         self._agent_id = self._config.get("agent_id", "hermes")
         self._rerank = self._config.get("rerank", True)