fix(eval): remove public preprocessor authoring

[christso]

Summary

Public authoring now has one output-shaping contract: use transform. The SDK no longer exports or serializes preprocessor helper options, defineEval() rejects top-level preprocessors, and the YAML loader no longer parses suite-level preprocessor config after rejecting authored usage.

The file-output example and current docs now present this as a transform workflow (examples/features/file-transforms) instead of a preprocessor example. Internal content conversion plumbing remains available only behind non-public runtime types, and retained source traceability uses content_transform_command terminology instead of preprocessor_command.

Validation

• bun install
• bun test packages/core/test/evaluation/preprocessors-yaml.test.ts packages/core/test/evaluation/transform-runtime.test.ts packages/core/test/evaluation/validation/eval-validator.test.ts packages/core/test/evaluation/validation/eval-file-schema.test.ts
• bun test packages/core/test/evaluation/source-traceability.test.ts packages/core/test/evaluation/orchestrator.test.ts packages/core/test/evaluation/llm-grader-multimodal.test.ts
• bun --filter @agentv/core typecheck
• bun --filter @agentv/sdk test
• bun --filter @agentv/sdk typecheck
• bun run validate:examples
• bun run lint
• bun --filter @agentv/sdk build

Live provider/grader dogfood was not rerun here because this PR hardens public authoring rejection and SDK/docs surfaces; the downstream transform docs/example dogfood gate remains tracked by av-kfik.16.

Related

Related: av-kfik.34.4

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