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)

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = ["wfe-core", "wfe-sqlite", "wfe-postgres", "wfe-opensearch", "wfe-valkey", "wfe"]
+members = ["wfe-core", "wfe-sqlite", "wfe-postgres", "wfe-opensearch", "wfe-valkey", "wfe", "wfe-yaml"]
 resolver = "2"
 
 [workspace.package]
@@ -41,6 +41,11 @@ wfe-sqlite = { path = "wfe-sqlite" }
 wfe-postgres = { path = "wfe-postgres" }
 wfe-opensearch = { path = "wfe-opensearch" }
 wfe-valkey = { path = "wfe-valkey" }
+wfe-yaml = { path = "wfe-yaml" }
+
+# YAML
+serde_yaml = "0.9"
+regex = "1"
 
 # Dev/Test
 pretty_assertions = "1"