fix(usage-log): serialise concurrent writes with per-file promise queue

Fire-and-forget callers (attempt.ts) can trigger two concurrent
recordTokenUsage() calls for the same workspaceDir. The previous
read-modify-write pattern had no locking, so the last writer silently
overwrote the first, losing that run's entry.

Fix: keep a Map<file, Promise<void>> write queue so each write awaits
the previous one. The queue slot is replaced with a no-throw wrapper so
a failed write does not stall future writes.

Added a concurrent-write test (20 parallel calls) that asserts no
record is lost.

src/agents/usage-log.test.ts

@@ -119,4 +119,25 @@ describe("recordTokenUsage", () => {
     expect(records[0].inputTokens).toBe(100);
     expect(records[0].outputTokens).toBe(50);
   });
+
+  it("serialises concurrent writes — no record is lost", async () => {
+    const N = 20;
+    await Promise.all(
+      Array.from({ length: N }, (_, i) =>
+        recordTokenUsage({
+          workspaceDir: tmpDir,
+          label: "llm_output",
+          usage: { input: i + 1, output: 1, total: i + 2 },
+        }),
+      ),
+    );
+
+    const records = JSON.parse(await fs.readFile(usageFile, "utf-8"));
+    expect(records).toHaveLength(N);
+    // Every distinct tokensUsed value must appear exactly once
+    const totals = records
+      .map((r: { tokensUsed: number }) => r.tokensUsed)
+      .toSorted((a: number, b: number) => a - b);
+    expect(totals).toEqual(Array.from({ length: N }, (_, i) => i + 2));
+  });
 });

src/agents/usage-log.ts

@@ -33,6 +33,17 @@ async function readJsonArray(file: string): Promise<TokenUsageRecord[]> {
   }
 }
 
+async function appendRecord(file: string, entry: TokenUsageRecord): Promise<void> {
+  const records = await readJsonArray(file);
+  records.push(entry);
+  await fs.writeFile(file, JSON.stringify(records, null, 2));
+}
+
+// Per-file write queue: serialises concurrent recordTokenUsage() calls so that
+// a fire-and-forget caller cannot cause two concurrent writers to read the same
+// snapshot and overwrite each other's entry.
+const writeQueues = new Map<string, Promise<void>>();
+
 export async function recordTokenUsage(params: {
   workspaceDir: string;
   runId?: string;
@@ -83,7 +94,11 @@ export async function recordTokenUsage(params: {
     createdAt: new Date().toISOString(),
   };
 
-  const records = await readJsonArray(file);
-  records.push(entry);
-  await fs.writeFile(file, JSON.stringify(records, null, 2));
+  const queued = writeQueues.get(file) ?? Promise.resolve();
+  const next = queued.then(() => appendRecord(file, entry));
+  writeQueues.set(
+    file,
+    next.catch(() => {}),
+  );
+  await next;
 }