Add retrieval telemetry imports for evaluations

[placerda]

Summary

• Adds the telemetry_imports configuration contract and agentops telemetry validate, preview, and import commands.
• Supports dataset-backed response reuse with response_source: dataset.
• Adds grey-box HTTP response field mapping from $response.context for retrieval telemetry.
• Updates evaluation documentation and changelog coverage for the retrieval telemetry import workflow.

Validation

• git diff --check
• PYTHONPATH=src python -m pytest tests/unit/test_agentops_config.py tests/unit/test_cli_commands.py tests/unit/test_http_response_fields.py tests/unit/test_runtime_dataset_response_source.py tests/unit/test_telemetry_import.py -q (77 passed)

Docs

• Updated README, concepts, evaluation docs, Foundry evaluator guide, and CHANGELOG.md.

Refs #145