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);
 }

packages/cli/src/__tests__/step-timing.test.ts

@@ -96,6 +96,7 @@ describe("protocol types", () => {
       completedAtMs: 2000,
       assembledPrompt: null,
       cwd: "/test/path",
+      usage: null,
     };
     expect(record.startedAtMs).toBe(1000);
     expect(record.completedAtMs).toBe(2000);
@@ -110,6 +111,7 @@ describe("protocol types", () => {
       agent: "uwf-test",
       timestamp: 123,
       durationMs: 5000,
+      usage: null,
     };
     expect(entry.durationMs).toBe(5000);
   });

packages/cli/src/commands/step.ts

@@ -66,6 +66,7 @@ export async function cmdStepList(
       agent: item.payload.agent,
       timestamp: item.timestamp,
       durationMs: item.payload.completedAtMs - item.payload.startedAtMs,
+      usage: item.payload.usage ?? null,
     });
   }