Agent bindings cannot override channel instance agent_id

Problem

The maintainer of PR #227 recommended using agent bindings for per-channel agent routing. However, bindings cannot work when a channel instance has an agent_id set.

Flow

1. Channel instance mother-slack has agent_id = <mother UUID>
2. Slack adapter creates message with msg.AgentID = "mother" (from BaseChannel.AgentID())
3. In processNormalMessage(), agentID := msg.AgentID → "mother"
4. resolveAgentRoute() is only called when agentID == "" — it never fires
5. Config bindings are completely bypassed

Code

// cmd/gateway_consumer_normal.go
agentID := msg.AgentID
if agentID == "" {
    agentID = resolveAgentRoute(cfg, msg.Channel, msg.ChatID, msg.PeerKind)
}

What I need

One Slack bot (Socket Mode) serving multiple agents. DMs → Mother. #cell-invoice → invoice agent. #cell-ops → ops agent.

Options I see

1. Make channel_instance.agent_id nullable — when NULL, msg.AgentID is empty, bindings kick in. Requires instance_loader to skip SetAgentID when agent_id is NULL.
2. Always check bindings first — before falling back to msg.AgentID. Peer-level bindings (channel + chat_id) are more specific than a channel-wide default.
3. Both — nullable agent_id + bindings-first.

Happy to submit a PR for whichever approach you prefer.