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/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "wfe-yaml"
+version.workspace = true
+edition.workspace = true
+description = "YAML workflow definitions for WFE"
+
+[dependencies]
+wfe-core = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+serde_yaml = { workspace = true }
+async-trait = { workspace = true }
+tokio = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true }
+regex = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+tokio = { workspace = true, features = ["test-util", "process"] }
+wfe-core = { workspace = true, features = ["test-support"] }
+wfe = { path = "../wfe" }