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

packages/workflow-role-committer/src/committer.ts

@@ -31,4 +31,5 @@ export const committerRole: RoleDefinition<CommitterMeta> = {
   extractPrompt:
     "Extract the commit result: committed (with branch and SHA), recoverable failure, or unrecoverable failure. Include error details and log references if applicable.",
   schema: committerMetaSchema,
+  extractRefs: null,
 };