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.

bench/README.md

@@ -0,0 +1,47 @@
+# Benchmark harness
+
+This directory holds the benchmark harness that measures gitea-axi's central claim — that it is an agent-ergonomic, low-token interface to Gitea — against the `tea` CLI, the official `gitea-mcp` server, and raw Gitea REST calls.
+
+It is **not part of the published npm package**.
+The package's `files` allow-list ships only `dist` and `skills`; `bench/` is excluded, and the packaging tier asserts it stays out of the tarball.
+
+The design lives in [`.claude/spec/benchmark-harness.md`](../.claude/spec/benchmark-harness.md) and the `.claude/adr/0014`–`0016` decision records.
+This README is the harness's own working documentation; it deliberately keeps the benchmark's vocabulary here rather than in the tool's domain glossary ([`.claude/CONTEXT.md`](../.claude/CONTEXT.md)), which describes gitea-axi's own language.
+
+## Vocabulary
+
+**arm** — one of the four tool conditions under comparison: `gitea-axi`, `tea`, `gitea-mcp`, `raw-api`.
+The comparison measures the tool, so the agent in each arm is given exactly one arm's tool.
+
+**cell** — one `(arm, task)` pair.
+A cell's trials accumulate as samples within it; deepening a cell's sample size adds samples rather than overwriting prior runs.
+
+**trial** — one run of a cell.
+Each cell defaults to five trials with a reporting floor of three.
+
+**tier** — the task category a task belongs to: `read`, `single-mutation`, `find-then-act`, `multi-step`.
+Views group by tier to show where an arm wins or loses.
+
+**cost-equivalent tokens** — the headline metric: the four token components weighted by Anthropic's published API pricing ratios (see ADR 0014).
+The raw component breakdown is retained on every sample so the data can be re-weighted without re-running.
+
+**seed** — the deterministic, idempotent starting state scripted into each throwaway repository before a trial, against which correctness is scored.
+
+**checker** — the deterministic scorer that diffs post-run repository state (mutation tasks) or matches required facts in the agent's report (read tasks) against the seeded ground truth.
+
+## Layout
+
+- `result.ts` — the immutable result-record shape and its tags (arm, task, tier, trial, timestamp).
+- `store.ts` — the append-only, per-cell sample store that accumulates result records.
+
+Later slices add the tool-isolation guard, the seed provisioning, the arm scaffolding, the single-cell runner, the task suite, the run-loop CLI, and the aggregator.
+
+## Tests
+
+The harness's deterministic seams are unit-tested in this directory, colocated with their source, and run via:
+
+```
+npm run test:bench
+```
+
+They are kept out of the main fast tier so harness code never counts against the `src/` coverage thresholds.