feat: add benchmark scaffold and result store (task 0022)

Lay the foundation the benchmark harness reads and writes: a bench/
directory (excluded from the published npm package), the immutable
result-record shape, and an append-only per-cell sample store.

- bench/result.ts: the ResultRecord shape — four token components,
  turns, duration, imputed cost, tagged pass/fail outcome, and the
  arm/task/tier/trial/timestamp tags. Arm and Tier are typed unions.
- bench/store.ts: append-only sample store, one JSONL file per cell at
  <root>/<arm>/<taskId>.jsonl; deepening a cell only ever adds samples.
- bench/README.md: harness working docs and benchmark vocabulary, kept
  out of the tool's domain glossary per the spec.
- Dedicated bench test tier (vitest.bench.config.ts, npm run test:bench)
  kept out of the fast tier; tsconfig typechecks bench.
- Packaging tier asserts bench/ never ships in the tarball.

test/packaging/packaging.test.ts

@@ -103,6 +103,10 @@ describe("npm distribution artifact", () => {
     expect(scripts?.postinstall).toBeUndefined();
   });
 
+  it("excludes the bench/ harness directory from the package", () => {
+    expect(existsSync(join(extractDir, "package", "bench"))).toBe(false);
+  });
+
   it("declares complete metadata: unscoped name, description, repo, license, engines, ESM type", () => {
     const name = packedManifest.name as string;
     expect(name).toBe("gitea-axi");