Docs/observability surfacedocs: document the current observability surface

[qsxns]

Closes #45
Closes #122

Summary

Documents the current observability surface of AReno — what training
emits through console logs, train_stats, TensorBoard scalar metrics,
and agentic trajectory diagnostics, and how to inspect each during
debugging.

This is a docs-only change. No code, dependencies, or CLI surface are
modified.

What's documented

docs/reference/environment-variables.rst

• ARENO_LOG_LEVEL — controls the areno logger level (default INFO).
  Set to DEBUG for per-request decode progress and fine-grained engine
  diagnostics.
• ARENO_LOG_COMPLETIONS — number of decoded rollout samples persisted per
  training step (default 1), written to rollout_samples.{pid}.jsonl.

docs/cli/observability.rst

• Console logs section now covers log-level control via ARENO_LOG_LEVEL.
• New "Reading metrics during debugging" section with an
  EventAccumulator code example for reading TensorBoard scalars
  programmatically.
• New "Dashboard" section covering areno dashboard --start/--stop, the
  default port, and the metrics-directory artifacts
  (dashboard_state.{pid}.json, rollout_samples.{pid}.jsonl,
  areno_run_config.{pid}.json / .txt).

The existing content already cited areno.api.metrics, --metrics-log-dir,
trainer logging paths, and the rollout/*, train/*, time/* namespaces,
and explicitly notes that AReno does not yet provide a built-in wandb
integration. All of that is preserved.

Acceptance criteria

• 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
• Documents logs, train_stats, TensorBoard, and agentic trajectory diagnostics
• Covers rollout/*, train/*, and time/* metric namespaces
• Explains how to launch TensorBoard for a run

Verification

• All 17 acceptance-criteria checks pass.
• RST heading structure validated — no underline-length errors.
• Every factual claim cross-checked against the source
  (areno/cli/dashboard.py, areno/cli/train.py, areno/api/metrics.py,
  areno/api/defaults.py).
• The dashboard CLI syntax in the docs was corrected to match the real
  interface (--start/--stop, no --no-browser).
• Sphinx is not installed locally, so make html was not run; the RST
  follows existing patterns in the same file.

Notes

• docs/cli/observability.rst remains an orphan page (not linked from the
  toctree) — consistent with the existing cross-references from
  docs/reference/cli.rst and docs/cli/training.rst.

[qsxns]

#45