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.

tsconfig.json

@@ -11,5 +11,5 @@
     "types": ["node"],
     "noEmit": true
   },
-  "include": ["src", "test", "vitest.config.ts"]
+  "include": ["src", "test", "bench", "vitest.config.ts", "vitest.bench.config.ts"]
 }