docs: add benchmark harness spec, ADRs, and task breakdown

Add the benchmark-harness spec, three supporting ADRs (cost-equivalent
token metric, single-user seed, guard-based tool isolation), and the
0022-0030 task breakdown that slices the harness into foundational
seams, an integrating single-cell runner, and the reporting layer.

.claude/tasks/0022-bench-scaffold-and-result-store.md

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+---
+spec: benchmark-harness
+---
+
+## What to build
+
+The foundation the whole benchmark harness reads and writes: a `bench/` directory (excluded from the published npm package, alongside the existing `dist`/`skills` allow-list), the immutable result-record shape, and an append-only per-cell sample store.
+
+A result record captures one completed `(arm, task, trial)` run: the four token components (fresh input, cache-creation, cache-read, output), the turn count, the wall-clock duration, the imputed cost, and the pass/fail outcome with a failure tag distinguishing a confused agent from a hung one. It also carries the tags later views group by — arm, task id, tier, trial, and a timestamp.
+
+The store appends records as immutable, timestamped samples to a per-cell location. Deepening a cell's sample size adds samples rather than overwriting any prior run, and reading a cell returns every accumulated sample.
+
+## Acceptance criteria
+
+- [ ] A `bench/` directory exists and is excluded from the npm package (verified by the packaging tier or an equivalent `files` check).
+- [ ] The result-record shape records the four token components, turns, duration, imputed cost, outcome, failure tag, and the arm/task/tier/trial/timestamp tags.
+- [ ] Appending a sample to a cell that already has samples leaves the prior samples intact; reading the cell returns all of them.
+- [ ] A round-trip test writes several samples across cells and reads back exactly what was written.