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/0015-bench-single-user-seed.md

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+# Single-user seed and its constraints on the task surface
+
+The benchmark seeds a throwaway repository to a known state before each trial and scores tasks against that ground truth.
+Seeding realistic author and assignee variety would require several Gitea accounts.
+The maintainer prefers to run the benchmark under their single existing account rather than provision additional accounts.
+
+## Considered Options
+
+**Provision throwaway collaborator accounts** (rejected) — Multiple accounts would restore the author and assignee dimensions and enable non-self pull-request approvals, but they add account lifecycle and credential handling that the maintainer explicitly declined for this benchmark.
+
+**Keep multi-user tasks and let arms fail** (rejected) — Retaining tasks that need distinct authors or a non-self reviewer would make those tasks impossible under one account for every arm uniformly, producing no comparative signal while consuming budget.
+
+**Single-user seed with a redesigned surface** (chosen) — All seed content is authored by the one account, and the discriminating dimensions become label, state, assignee presence (assigned-to-self versus unassigned), and title keyword instead of author.
+Tasks that assumed author or assignee variety are recast onto these axes: reassignment becomes assign-to-self or unassign, and author-filtered bulk mutation becomes a filter on assignee presence.
+
+## Consequences
+
+- Author filtering leaves the scored suite; it carries little signal with one author anyway.
+- Review tasks in the scored suite use comment-type reviews, which a user may leave on their own pull request.
+  Whether the host permits self-approval and self-request-changes is verified during implementation; if permitted, those two tasks are promoted from comment reviews to approve and request-changes, otherwise approve and request-changes move to the bonus table as two-account scenarios.
+- Non-self approval, distinct-author filtering, and distinct-assignee tasks are out of scope for the scored suite and belong to the multi-account bonus scenarios.
+- The seed stays small and fully deterministic, which keeps the full-state diff used for collateral-damage checking cheap to compute and reason about.