Document the current observability surface

[adohe]

Spec candidate: O1.

Scope

• Document what AReno currently emits through logs, train_stats, TensorBoard, and agentic trajectory diagnostics.
• Include metric namespaces such as rollout/*, train/*, and time/*.
• Explain how to launch TensorBoard for a run.

Acceptance

• Cites areno.api.metrics, --metrics-log-dir, and trainer logging paths.
• Does not claim wandb support yet.
• Includes at least one example of reading a metric during debugging.