feat: add $usage field to adapter protocol

- Add Usage type to protocol (turns, inputTokens, outputTokens, duration)
- Add usage to StepRecord, StepNodePayload, StepEntry, STEP_NODE_SCHEMA
- Thread usage through util-agent extract pipeline (writeStepNode → persistStep → createAgent)
- All adapters return usage: null as placeholder (mock, hermes, claude-code, builtin)
- 746 tests pass, no breaking changes (usage not in schema required array)

Fixes #74
Refs #68

packages/protocol/src/index.ts

@@ -44,6 +44,7 @@ export type {
   ThreadStatus,
   ThreadStepsOutput,
   ThreadsIndex,
+  Usage,
   WorkflowConfig,
   WorkflowName,
   WorkflowPayload,