refactor: remove main session key references and enhance agent-gateway associations

backend/app/models/agents.py

@@ -22,6 +22,7 @@ class Agent(QueryModel, table=True):
 
     id: UUID = Field(default_factory=uuid4, primary_key=True)
     board_id: UUID | None = Field(default=None, foreign_key="boards.id", index=True)
+    gateway_id: UUID = Field(foreign_key="gateways.id", index=True)
     name: str = Field(index=True)
     status: str = Field(default="provisioning", index=True)
     openclaw_session_id: str | None = Field(default=None, index=True)

backend/app/models/gateways.py

@@ -23,7 +23,6 @@ class Gateway(QueryModel, table=True):
     name: str
     url: str
     token: str | None = Field(default=None)
-    main_session_key: str
     workspace_root: str
     created_at: datetime = Field(default_factory=utcnow)
     updated_at: datetime = Field(default_factory=utcnow)