Support external commands for custom jobs #27

Context

Allow custom transformers written in other languages while keeping pull-up responsible for collecting inputs, comparing results, and writing outputs.

This builds on YAML configuration support.

Proposed work

#26 must be implemented before this.

Add type: custom with an executable and argument array:

jobs:
  ownership:
    type: custom
    command: "python3 scripts/ownership.py"
    input: ["packages/**/ownership.yaml"]
    output: ".github/CODEOWNERS"

Run one process per custom job from the repository root, without shell interpretation. Send a single UTF-8 JSON document through stdin:

{
  "protocolVersion": 1,
  "sources": [
    {
      "path": "packages/example/ownership.yaml",
      "contents": "owners: [\"@example-team\"]\n"
    }
  ],
  "context": {
    "rootDir": "/workspace/repo",
    "outputPath": "/workspace/repo/.github/CODEOWNERS"
  }
}

Source paths are repository-relative; context paths are absolute. Stdout contains generated contents, and stderr contains diagnostics. Transformers return contents without modifying files directly; consuming repositories provide the required runtimes.

Acceptance criteria

• Each sync, check, or sync --dry-run invocation runs the transformer. Only sync writes changed outputs.
• Deduplicate sources and sort paths by UTF-8 bytes. Preserve contents exactly, allow empty sources/output, and reject invalid UTF-8.
• Report startup, stdin delivery, nonzero exit, signal, and output-read failures. Accept results only after complete input delivery and output collection.
• Complete all input collection and transformations before writing outputs. Failures or cancellation leave outputs unchanged and clean up active processes.
• Document migration from callback-based jobs and provide a working transformer example.