Devcontainers: When there is one parent agent and one subagent (devcontainer), `coder ssh/port-forward/etc` should hop into the devcontainer rather than select randomly between parent and sub

[mafredri]

In the Web UI, we prioritize dev containers over the workspace whenever there are dev containers. Likewise, we should prioritize dev containers for coder ssh, coder port-forward, etc.

In the cli package there is a method for workspace/agent selection which currently uses randomness to pick one agent. We can simply amend this code-path to make a stable selection of the agent.

There is should never be a reason to select the agent randomly.

If there are multiple subagents (dev containers), we should error out and ask the user to specify the agent explicitly when connecting.

This could potentially be marked as a breaking change, although it's unclear if someone relying on the randomness could be considered a feature.

Notes

• Consider breaking change
• Coder Desktop compatibility
  • Currently CD disables short domains when there are multiple agents, we could add a custom case for allowing the 1 sub agent scenario