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.

gateway/run.py

@@ -4551,6 +4551,7 @@ class GatewayRunner:
                     provider_data_collection=pr.get("data_collection"),
                     session_id=task_id,
                     platform=platform_key,
+                    user_id=source.user_id,
                     session_db=self._session_db,
                     fallback_model=self._fallback_model,
                 )
@@ -6645,6 +6646,7 @@ class GatewayRunner:
                     provider_data_collection=pr.get("data_collection"),
                     session_id=session_id,
                     platform=platform_key,
+                    user_id=source.user_id,
                     session_db=self._session_db,
                     fallback_model=self._fallback_model,
                 )