feat(yaml-parser): add input_files shorthand to EVAL.yaml test cases

[christso]

Summary

• Adds input_files: as a shorthand at the test level in EVAL.yaml that expands to type: file content blocks prepended before a type: text block in the user message
• Implementation lives entirely in the parser/loader layer (shorthand-expansion.ts) — no provider or evaluator changes
• Only supported with a string input in v1; multi-turn array inputs return undefined (no expansion)
• Paths are resolved using the same convention as existing explicit type: file blocks

Expansion rule:

# shorthand
input_files:
  - evals/files/sales.csv
input: "Summarize the monthly trends in this CSV."

expands to:

input:
  - role: user
    content:
      - type: file
        value: evals/files/sales.csv
      - type: text
        value: "Summarize the monthly trends in this CSV."

Changes

• packages/core/src/evaluation/loaders/shorthand-expansion.ts — new expandInputFilesShorthand() function; updated resolveInputMessages() to check input_files first
• packages/core/src/evaluation/yaml-parser.ts — added input_files?: JsonValue to RawEvalCase type
• packages/core/test/evaluation/loaders/shorthand-expansion.test.ts — 8 new unit tests for expandInputFilesShorthand, 4 new tests for resolveInputMessages with input_files
• packages/core/test/evaluation/input-files-shorthand.test.ts — 4 new yaml-parser integration tests (including parity test between shorthand and explicit form)
• examples/features/input-files-shorthand/ — new example eval demonstrating single-file, multi-file, and explicit-form equivalence

Risk

Low — purely additive parser change. No existing behavior changes. All 1300 tests pass.

closes #602