Devcontainers: Fix passing of SubAgentClient as it breaks when the agents reconnects the DRPC API

[mafredri]

The implementation initializes the SubAgentClient (agent DRPC API) for agentcontainers.API during agent API setup. This is incorrect as the client changes on disconnect/reconnect and needs to be reflected.

While we're at it, we should also ensure that agentcontainers.API waits and does an immediate refresh of containers when the agent startup has completed (this improves responsiveness and avoids injecting the sub agent too early).