Decide the wandb integration shape

[adohe]

Spec candidate: O4.

Scope

• Decide whether wandb should be supported through an optional sink, an env flag, a CLI flag, or a later plugin-style integration.
• Compare the choice against existing TensorBoard behavior.
• Defer implementation unless maintainers approve the API/CLI surface.

Acceptance

• Explicitly states wandb must remain optional.
• Calls out whether a future change needs kind/api-change or area/dependency.
• No runtime dependency is added in this issue.