feat(wfe-yaml): add YAML workflow definitions with shell executor

Concourse-CI-inspired YAML format for defining workflows. Compiles
to standard WorkflowDefinition + step factories.

Features:
- Schema parsing with serde_yaml (YamlWorkflow, YamlStep, StepConfig)
- ((var.path)) interpolation from config maps at load time
- YAML anchors (&anchor/*alias) fully supported
- Validation at load time (no runtime surprises)
- Shell executor: runs commands via tokio::process, captures stdout,
  parses ##wfe[output name=value] annotations for structured outputs
- Compiler: sequential wiring, parallel blocks, on_failure/on_success/
  ensure hooks, error behavior mapping
- Public API: load_workflow(), load_workflow_from_str()
- 23 tests (schema, interpolation, compiler, e2e)

wfe-yaml/src/lib.rs

@@ -0,0 +1,38 @@
+pub mod compiler;
+pub mod error;
+pub mod executors;
+pub mod interpolation;
+pub mod schema;
+pub mod validation;
+
+use std::collections::HashMap;
+
+use crate::compiler::CompiledWorkflow;
+use crate::error::YamlWorkflowError;
+
+/// Load a workflow from a YAML file path, applying variable interpolation.
+pub fn load_workflow(
+    path: &std::path::Path,
+    config: &HashMap<String, serde_json::Value>,
+) -> Result<CompiledWorkflow, YamlWorkflowError> {
+    let yaml = std::fs::read_to_string(path)?;
+    load_workflow_from_str(&yaml, config)
+}
+
+/// Load a workflow from a YAML string, applying variable interpolation.
+pub fn load_workflow_from_str(
+    yaml: &str,
+    config: &HashMap<String, serde_json::Value>,
+) -> Result<CompiledWorkflow, YamlWorkflowError> {
+    // Interpolate variables.
+    let interpolated = interpolation::interpolate(yaml, config)?;
+
+    // Parse YAML.
+    let workflow: schema::YamlWorkflow = serde_yaml::from_str(&interpolated)?;
+
+    // Validate.
+    validation::validate(&workflow.workflow)?;
+
+    // Compile.
+    compiler::compile(&workflow.workflow)
+}