feat(workflow): ExecutionPlanCompiler + ExecutionPlanLockedEvent (Slice 3)

- ExecutionPlanLockedEvent: registered, replay-deterministic record of the
  locked plan (CAS ref + compiled stage/start ids).
- ExecutionPlanCompiler: validated execution_plan JSON -> WorkflowGraph,
  reusing ConditionSpec.toCondition(); inline prompts via metadata[promptInline];
  malformed plans throw WorkflowValidationException.
- Replay test: locked event + plan JSON recompiles to the identical graph,
  no architect re-run, no FS reads.

testing/replay/build.gradle

@@ -14,4 +14,6 @@ dependencies {
     testImplementation(project(":core:approvals"))
     testImplementation(project(":testing:fixtures"))
     testImplementation(project(":infrastructure:persistence"))
+    testImplementation(project(":infrastructure:workflow"))
+    testImplementation(project(":core:artifacts"))
 }