SubWorkflowStep was hard-coding `inputs: serde_json::Value::Null` from
the YAML compiler, so every `type: workflow` step kicked off a child
instance with an empty data object. Scripts in child workflows then
saw empty `$REPO_URL`, `$COMMIT_SHA`, etc. and failed immediately.
Now: when no explicit inputs are set, the child inherits the parent
workflow's data (when it's an object). Scripts in child workflows can
reference the same top-level inputs the parent was started with without
every `type: workflow` step needing to re-declare them.
LogSink trait for real-time step output streaming. Added to
StepExecutionContext as optional field (backward compatible).
Threaded through WorkflowExecutor and WorkflowHostBuilder.
Wired LifecyclePublisher.publish() into executor at 5 points:
StepStarted, StepCompleted, Error, Completed, Terminated.
Also added lifecycle events to host start/suspend/resume/terminate.
SchemaType enum with inline syntax parsing: "string", "string?",
"list<number>", "map<string>", nested generics. WorkflowSchema
validates inputs/outputs against type declarations at both compile
time and runtime. 39 tests for parse and validate paths.
