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/error.rs

@@ -0,0 +1,13 @@
+#[derive(Debug, thiserror::Error)]
+pub enum YamlWorkflowError {
+    #[error("YAML parse error: {0}")]
+    Parse(#[from] serde_yaml::Error),
+    #[error("Interpolation error: unresolved variable '{0}'")]
+    UnresolvedVariable(String),
+    #[error("Validation error: {0}")]
+    Validation(String),
+    #[error("Compilation error: {0}")]
+    Compilation(String),
+    #[error("IO error: {0}")]
+    Io(#[from] std::io::Error),
+}