fix(eval): migrate authored expected output to vars

[christso]

Summary

Authored Promptfoo-aligned YAML now treats expected_output as removed at the top level, in default_test, and in inline tests[] rows. Reference answers belong in vars.expected_output, where they are inert unless an authored assertion or grader explicitly consumes {{ expected_output }}.

The hard-deprecation codemod now migrates legacy authored references into vars.expected_output, preserving existing assertion/criteria strategies and adding a reference-matching llm-rubric only when the legacy case had no explicit grading strategy. Examples and local fixtures have been migrated to the supported shape, and the generated eval schema reflects that authored YAML expected_output is no longer accepted.

The TypeScript SDK materialization bridge keeps carrying internal expected_output payloads for now, so this PR stays within the authored YAML slice and leaves SDK/API compatibility cleanup to av-kfik.28.4.

Related: av-kfik.28.2

Validation

• bun --filter @agentv/core test
• bun --filter agentv test
• bun test packages/core/test/evaluation/loaders/jsonl-parser.test.ts packages/core/test/evaluation/validation/eval-validator.test.ts scripts/migrate-hard-deprecations.test.ts packages/core/test/evaluation/criteria-optional.test.ts packages/core/test/evaluation/suite-level-input.test.ts packages/core/test/evaluation/conversation-mode.test.ts packages/core/test/evaluation/validation/eval-schema-sync.test.ts packages/core/test/evaluation/validation/eval-file-schema.test.ts
• bun test packages/core/test/evaluation/eval-inline-experiment.test.ts
• bun test apps/cli/test/commands/prepare/prepare.test.ts apps/cli/test/eval.integration.test.ts
• bun test apps/cli/test/commands/runs/rerun.test.ts apps/cli/test/commands/grade/grade-prepared.test.ts
• bun --filter @agentv/core build
• bun --filter @agentv/sdk build
• bun run generate:schema
• bun run validate:examples
• bun run lint
• git diff --check

Live provider dogfood was not run because this slice changes authored YAML loading, validation, schema, codemod, and examples; it does not change provider execution, LLM grader execution, or run artifact layout.

Post-Deploy Monitoring & Validation

No additional production monitoring required. This is a local authoring/schema/codemod change with no deployed service path; CI validation and example validation are the release signals.

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