PR 12: Retire benchmark authoring indirection

[cm2435]

Summary

Retires the legacy benchmark authoring layer in favor of the RFC's Experiment -> Environment -> Sample path. Builtin examples now compose data loaders, explicit workers/evaluators/sandboxes, and make_*_sample(...) helpers through Environment.from_dataset(...) / Environment.from_records(...).

What changed

• Added row-backed Environment.from_dataset(...) and Environment.from_records(...) constructors.
• Replaced bespoke builtin environment classes with benchmark-local dataset.py, sample.py, and concrete task.py helpers.
• Deleted thin worker_factory.py, _default_*_sandbox, benchmark class, and variable-alias compatibility layers.
• Ported examples, smoke fixtures, real-LLM harnesses, and builtin unit tests to explicit Pythonic composition.
• Renamed CLI benchmark setup surfaces to environment setup to match the public vocabulary.
• Tightened the remaining cleanup polish: Pydantic preflight result, clearer row-backed test fixture names, no name-to-name aliases.

Why

The old builtin path hid the important authoring choices behind factories and bespoke environment classes. That made examples harder to inspect and conflicted with the RFC goal of having one public composition shape for users and builtins. This PR makes the examples read directly: load rows, construct worker/evaluator/sandbox objects, build samples, submit an experiment.

Review notes / gotchas

• make_*_sample(...) helpers remain intentionally: they are row-to-sample builders, not thin object aliases.
• load_*_rows(...) / iter_*_rows(...) remain intentionally as source helpers.
• The removed environment classes are guarded by tests so they do not silently re-enter the public API.
• This PR is stacked on PR 11 (codex/episode-sample-pr11-delete-definition-runtime), not directly on dev.

Validation

• uv run pytest ergon_builtins/tests/unit/environments tests/examples ergon_core/tests/unit/api/test_environment_constructors.py ergon_builtins/tests/unit/benchmarks/test_source_sample_helpers.py ergon_core/tests/unit/read_models/test_sample_read_service.py ergon_core/tests/unit/runtime/test_execute_task_evaluator_fanout.py ergon_core/tests/unit/core/application/jobs/test_worker_execute_live_sandbox_attach.py -q
• uv run ruff check examples/getting_started/_shared/env.py ergon_builtins/tests/unit/environments/conftest.py tests/fixtures/smoke_components/__init__.py ergon_core/ergon_core/core/application/runtime/sample_records.py ergon_core/ergon_core/core/application/samples/event_views.py ergon_core/ergon_core/core/jobs/task/worker_execute/contract.py tests/e2e/_asserts.py ergon_core/tests/unit/read_models/test_sample_read_service.py ergon_core/ergon_core/core/jobs/task/execute/job.py ergon_core/ergon_core/core/jobs/task/worker_execute/job.py ergon_core/tests/unit/core/application/jobs/test_worker_execute_live_sandbox_attach.py ergon_core/tests/unit/runtime/test_execute_task_evaluator_fanout.py

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[cm2435]

Closed after landing the cumulative episode/sample stack snapshot through PR132 directly onto dev in merge commit 1c22130. This PR's changes are included in dev.