feat: add refs tracking to RoleStep

- RoleOutput gains refs: string[] for CAS reference tracking
- RoleDefinition gains extractRefs: ((meta) => string[]) | null
- planner: phases.map(p => p.hash), coder: [completedPhase]
- Engine persists refs, fork preserves refs
- Backward compat: missing refs normalized to []
- 137 tests passing

Fixes #31

examples/hello-world.ts

@@ -28,6 +28,7 @@ const greeter: RoleDefinition<Roles["greeter"]> = {
   systemPrompt: "You greet the user briefly.",
   extractPrompt: "Extract the greeting string produced for the user.",
   schema: greeterMetaSchema,
+  extractRefs: null,
 };
 
 const extract = createExtract({