fix: run the bench agent in a neutral working directory

The SDK driver ran the agent with no explicit cwd, so its shell inherited
the harness's own checkout. When the agent omitted `-R OWNER/NAME`, the
gitea-axi (and tea) CLI defaulted the repository from that local checkout —
silently resolving the harness repo instead of the seeded throwaway — and
returned a plausible but wrong result (e.g. `count: 0 open of 0 total` for a
repo with no issues). This contaminated read-tier scoring for the checkout-
defaulting arms and was surfaced by the newly persisted read reports.

Give each run a fresh, empty working directory outside any checkout, so a
forgotten `-R` errors instead of hitting the wrong repository, and delete it
when the run ends.

bench/sdk-driver.test.ts

@@ -1,5 +1,7 @@
+import { readdirSync, rmSync, statSync } from "node:fs";
+import path from "node:path";
 import { describe, expect, it } from "vitest";
-import { sumTokens } from "./sdk-driver.js";
+import { createAgentWorkdir, sumTokens } from "./sdk-driver.js";
 import type { SdkResultMessage } from "./sdk-driver.js";
 
 describe("sumTokens", () => {
@@ -87,3 +89,31 @@ describe("sumTokens", () => {
     });
   });
 });
+
+describe("createAgentWorkdir", () => {
+  // Behavior: each agent run must operate in a fresh, empty directory located
+  // OUTSIDE the harness's own checkout. This isolation is what stops an agent
+  // that forgets an explicit `-R OWNER/NAME` from having its `gitea-axi`/`tea`
+  // tools silently default their target repo to the harness's own git checkout:
+  // a directory that is empty (no `.git`) and outside the current checkout gives
+  // those tools nothing local to resolve.
+  //
+  // The three assertions are independent anti-bug properties drawn directly from
+  // the requirement, not recomputed from the implementation:
+  //   1. the path exists and is a directory,
+  //   2. it is empty (zero entries — in particular no `.git`),
+  //   3. it sits outside the current working directory (relative path escapes
+  //      upward with "..").
+  it("returns a fresh, empty directory located outside the current checkout", () => {
+    const dir = createAgentWorkdir();
+    try {
+      expect(statSync(dir).isDirectory()).toBe(true);
+      expect(readdirSync(dir)).toHaveLength(0);
+      expect(path.relative(process.cwd(), dir).startsWith("..")).toBe(true);
+    } finally {
+      // Safe: `dir` is a fresh throwaway temp dir we just received from
+      // createAgentWorkdir(); never a delete of cwd or any pre-existing path.
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+});