tui: show token usage breakdown in sidebar for transparency

Adds input, output and cached token counts to session sidebar so users
can monitor their consumption while working.

packages/opencode/test/cli/tui/usage.test.ts

@@ -0,0 +1,61 @@
+// kilocode_change - new file
+import { describe, expect, test } from "bun:test"
+import type { AssistantMessage, Message, UserMessage } from "@kilocode/sdk/v2"
+import { formatCount, getUsage } from "../../../src/cli/cmd/tui/routes/session/usage"
+
+function assistant(id: string, input: number, output: number, read: number): AssistantMessage {
+  return {
+    id,
+    sessionID: "ses_1",
+    role: "assistant",
+    time: { created: 1 },
+    parentID: "msg_parent",
+    modelID: "claude-sonnet",
+    providerID: "anthropic",
+    mode: "code",
+    agent: "code",
+    path: { cwd: "/tmp", root: "/tmp" },
+    cost: 0,
+    tokens: {
+      input,
+      output,
+      reasoning: 99,
+      cache: {
+        read,
+        write: 7,
+      },
+    },
+  }
+}
+
+function user(): UserMessage {
+  return {
+    id: "msg_user",
+    sessionID: "ses_1",
+    role: "user",
+    time: { created: 0 },
+    agent: "code",
+    model: {
+      providerID: "anthropic",
+      modelID: "claude-sonnet",
+    },
+  }
+}
+
+describe("session usage", () => {
+  test("sums input, output, and cache read across assistant messages only", () => {
+    const msg: Message[] = [user(), assistant("a", 1200, 45, 300), assistant("b", 800, 55, 700)]
+
+    expect(getUsage(msg)).toEqual({
+      input: 2000,
+      output: 100,
+      cached: 1000,
+    })
+  })
+
+  test("formats full counts with thousands separators", () => {
+    expect(formatCount(0)).toBe("0")
+    expect(formatCount(12345)).toBe("12,345")
+    expect(formatCount(9876543)).toBe("9,876,543")
+  })
+})