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/cli/src/__tests__/step-show-json.test.ts

@@ -118,6 +118,7 @@ async function createTestStep(
     completedAtMs: Date.now() + 1000,
     assembledPrompt: null,
     cwd: "/tmp",
+    usage: null,
   };
   return store.cas.put(schemas.stepNode, stepPayload);
 }