terraform-provider-coder: Add `subagent_id` computed field to `coder_devcontainer` resource

Add a new computed field `subagent_id` to the `coder_devcontainer` resource in the Terraform provider. This field will expose the ID of the Coder agent used by the dev container, allowing it to be referenced in `coder_app`, `coder_script`, `coder_env`, and `coder_ai_task` resources.