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/schema.rs

@@ -0,0 +1,72 @@
+use std::collections::HashMap;
+
+use serde::Deserialize;
+
+#[derive(Debug, Deserialize)]
+pub struct YamlWorkflow {
+    pub workflow: WorkflowSpec,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct WorkflowSpec {
+    pub id: String,
+    pub version: u32,
+    #[serde(default)]
+    pub description: Option<String>,
+    #[serde(default)]
+    pub error_behavior: Option<YamlErrorBehavior>,
+    pub steps: Vec<YamlStep>,
+    /// Allow unknown top-level keys (e.g. `_templates`) for YAML anchors.
+    #[serde(flatten)]
+    pub _extra: HashMap<String, serde_yaml::Value>,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct YamlStep {
+    pub name: String,
+    #[serde(rename = "type")]
+    pub step_type: Option<String>,
+    #[serde(default)]
+    pub config: Option<StepConfig>,
+    #[serde(default)]
+    pub inputs: Vec<DataRef>,
+    #[serde(default)]
+    pub outputs: Vec<DataRef>,
+    #[serde(default)]
+    pub parallel: Option<Vec<YamlStep>>,
+    #[serde(default)]
+    pub error_behavior: Option<YamlErrorBehavior>,
+    #[serde(default)]
+    pub on_success: Option<Box<YamlStep>>,
+    #[serde(default)]
+    pub on_failure: Option<Box<YamlStep>>,
+    #[serde(default)]
+    pub ensure: Option<Box<YamlStep>>,
+}
+
+#[derive(Debug, Deserialize, Clone)]
+pub struct StepConfig {
+    pub run: Option<String>,
+    pub file: Option<String>,
+    pub script: Option<String>,
+    pub shell: Option<String>,
+    #[serde(default)]
+    pub env: HashMap<String, String>,
+    pub timeout: Option<String>,
+    pub working_dir: Option<String>,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct DataRef {
+    pub name: String,
+    pub path: Option<String>,
+    pub json_path: Option<String>,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct YamlErrorBehavior {
+    #[serde(rename = "type")]
+    pub behavior_type: String,
+    pub interval: Option<String>,
+    pub max_retries: Option<u32>,
+}