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/agent-mock/src/mock-agent.ts

@@ -103,6 +103,7 @@ export function createMockAgent(mockDataPath: string): () => Promise<void> {
       detailHash,
       sessionId,
       assembledPrompt: "",
+      usage: null,
     };
     lastResult = result;
     return result;