Support YAML block object inputs

[gregreindel]

Object-shaped inputs like data, parser-options, llm-options, and executor-options currently need to be provided as JSON strings:

with:
  data: |
    {
      "input": "hello",
      "count": 3
    }

This works, but it feels awkward inside GitHub Actions YAML and makes simple examples look more technical than they need to be.

Proposal: accept YAML-formatted object strings for the same inputs, while keeping JSON support unchanged.

with:
  data: |
    input: hello
    count: 3

This would still be parsed from a string, because GitHub Actions with: inputs are scalar strings. It would not support true nested with: maps like:

with:
  data:
    input: hello

Suggested behavior:

• Empty input still defaults to {}.
• Valid JSON object strings continue to work.
• Valid YAML object strings also work.
• Arrays/scalars are rejected with a clear error.
• Applies consistently to data, parser-options, llm-options, and executor-options.

Why:

• Makes the action feel more native to GitHub Actions.
• Improves copy/paste ergonomics for non-expert users.
• Supports the fast setup value prop without breaking existing workflows.
• Runtime impact should be negligible because the YAML parser is bundled into the action and input parsing is tiny compared with the LLM call.

Implementation sketch:

• Add an explicit YAML parser dependency, likely js-yaml.
• Replace readJsonObject with readObjectInput.
• Try JSON.parse first, then YAML parse.
• Keep the existing plain-object validation.
• Add tests for JSON, YAML object strings, empty defaults, and rejected arrays/scalars.
• Update README examples to prefer YAML blocks while noting JSON is also supported.