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/adr/0014-bench-cost-equivalent-token-metric.md

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+# Cost-equivalent tokens as the benchmark's headline metric
+
+The benchmark compares how much each arm costs to drive.
+The maintainer runs on a Claude subscription with a fixed weekly token allowance, so the scarce resource is token consumption against that allowance, not dollars.
+The question is how to reduce each run's four token components — fresh input, cache-creation, cache-read, and output — into a single headline number that reflects weekly-budget burn.
+
+Research into Anthropic's documentation established that the exact unit and per-component weighting of the subscription weekly limit are not publicly documented.
+The one anchoring signal is that overage past the included allowance is billed at standard API rates, which points toward cost-weighted accounting rather than a flat token count.
+
+## Considered Options
+
+**Raw summed tokens as headline** (rejected) — Summing all four components at 1× is transparent and assumption-free, but cache-read routinely dominates the total, and Anthropic's API prices cache-reads at roughly a tenth of fresh input.
+A raw sum therefore overstates the burn of arms whose context is largely cached (notably the eager-schema MCP arm) by up to an order of magnitude, which would misrank the arms on the very axis the benchmark exists to measure.
+
+**Imputed dollars as headline** (rejected) — The runtime already reports an imputed cost that folds in every component at the correct weights.
+It is an accurate comparative number, but it is expressed in a unit the maintainer does not spend; on a subscription no dollars leave the account, and the mental model is weekly tokens.
+
+**Cost-equivalent tokens as headline** (chosen) — Weight the four components by Anthropic's published API pricing ratios (fresh input 1×, cache-write 1.25× or 2× by TTL, cache-read 0.1×, output 5×) and express the result as a token count.
+This is tokens — the maintainer's unit — weighted the way their budget most plausibly burns, and it is the same ranking as the imputed dollar figure.
+
+## Consequences
+
+- The headline is cost-equivalent tokens; the raw summed tokens and the full four-component breakdown are recorded alongside every run, so the data can be re-weighted without re-running if the subscription's real accounting is ever documented.
+- Imputed dollars are retained as a de-emphasized secondary column, portable for readers who are on the API rather than a subscription.
+- The weighting is an explicit, documented assumption grounded in the overage-pricing signal, not a measured fact; an optional later validation could pin the real weekly weighting empirically by burning a known token mix.
+- The auxiliary small model invoked by the runtime is counted rather than suppressed, since it is real consumption against the same allowance.