fix(opencode): include cache writes in ACP usage (#40450)

Co-authored-by: Aiden Cline <rekram1-node@users.noreply.github.com>

packages/opencode/test/acp/usage.test.ts

@@ -207,7 +207,7 @@ describe("acp usage", () => {
     )
   })
 
-  it.effect("sends ACP usage_update with context size and cumulative assistant cost", () => {
+  it.effect("includes cache reads and writes in ACP context usage", () => {
     const updates: SessionNotification[] = []
     return Effect.gen(function* () {
       const usage = yield* UsageService.Service
@@ -222,7 +222,7 @@ describe("acp usage", () => {
           sessionId: "ses_1",
           update: {
             sessionUpdate: "usage_update",
-            used: 15,
+            used: 22,
             size: 128_000,
             cost: { amount: 3, currency: "USD" },
           },
@@ -239,7 +239,7 @@ describe("acp usage", () => {
                 input: 10,
                 output: 20,
                 reasoning: 0,
-                cache: { read: 5, write: 0 },
+                cache: { read: 5, write: 7 },
               },
             }),
           ]),