feat: AI transform executor with Ollama and OpenAI-compatible backend support

[Copilot]

Adds AI-powered transforms as first-class pipeline nodes, enabling use cases like markdown → Fountain screenplay via local or remote LLMs.

New: AiTransform (src/transforms/ai.rs)

• AiBackend::Ollama — POSTs to <endpoint>/api/generate (stream: false)
• AiBackend::OpenAi — POSTs to <endpoint>/v1/chat/completions
• Prompt templates with {input} placeholder substitution
• Optional Authorization: Bearer header for authenticated APIs
• Optional artifact_path to write output to a file after each call
• Error messages include raw response body for debuggability

Extended YAML schema (yaml_loader.rs)

New fields on YamlTransformDef: ai, model, prompt, endpoint, api_key, artifact_path. Dispatch priority: ai > plugin > program.

transforms:
  - name: md-to-screenplay
    ai: ollama
    model: llava
    prompt: "Convert this markdown to Fountain screenplay: {input}"
    endpoint: http://localhost:11434
    artifact_path: output/screenplay.fountain
    from: markdown
    to: fountain
    cost: 2.0
    quality: 0.85

Other

• Format::Fountain added to the format graph for screenplay output
• ureq = { version = "2", features = ["json"] } added for blocking HTTP (no async runtime required)

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