feat(experimentalist): add Harbor examples

[ngoncharenko]

Summary

It's a follow up for this thread: #955 (comment)

• add a deterministic hello-harbor-agent example with local train and validation tasks, exact-output verifiers, traces, and an optimizer profile
• document the Experimentalist architecture and the harbor_native / harbor_evaluator A/B workflow
• add focused baseline, evaluator A/B, cache, and Tau3 smoke-profile coverage

Why this is separate

This keeps the documentation, examples, and smoke tests independently reviewable and reversible from the evaluator implementation, as requested in this review comment.

The new hello example is also a fast smoke-test path:

• the baseline test needs no Docker, model API key, or network access
• the live A/B test needs Docker but no model API key
• the live test exercises both evaluator paths plus cache reuse and force_rerun

Stack

This PR targets #955 because the docs, configuration, and A/B tests intentionally reference the harbor_evaluator implementation introduced there. Once #955 lands, this becomes an examples-only follow-up to main.

Verification

• uv run --frozen pytest plugins/nemo-experimentalist/tests/experimentalist/test_evaluator_harbor_ab_e2e.py plugins/nemo-experimentalist/tests/experimentalist/test_hello_example_baseline.py plugins/nemo-experimentalist/tests/test_e2e_helper_defaults.py -q — 17 passed
• uv run --frozen ruff check plugins/nemo-experimentalist/examples plugins/nemo-experimentalist/tests/experimentalist/test_evaluator_harbor_ab_e2e.py plugins/nemo-experimentalist/tests/experimentalist/test_hello_example_baseline.py plugins/nemo-experimentalist/tests/test_e2e_helper_defaults.py
• uv run --frozen ruff format --check plugins/nemo-experimentalist/examples plugins/nemo-experimentalist/tests/experimentalist/test_evaluator_harbor_ab_e2e.py plugins/nemo-experimentalist/tests/experimentalist/test_hello_example_baseline.py plugins/nemo-experimentalist/tests/test_e2e_helper_defaults.py
• git diff --check

Refs AALGO-312