Function parse_workflow 

pub fn parse_workflow(yaml: &str) -> Result<Workflow, NikaError>

Parse a YAML workflow through the three-phase pipeline.

Pipeline: YAML → raw::parse → analyzer::analyze → lower → Workflow

Returns the runtime Workflow type consumed by expand_includes and Runner.

Errors

• NikaError::ParseError — YAML syntax or structural errors (Phase 1)
• NikaError::ValidationError — Semantic validation errors (Phase 2)