feat: add efficacy-benchmark tracer bullet (task 0004)

Introduce the thinnest complete path that benchmarks a skill's efficacy
against a no-skill baseline and renders an HTML report.

- tests/ convention: a top-level tree mirroring skills/ by full path, with
  case directories holding a case.md (CASE-FORMAT.md) and an optional fixture.
- benchmark-skill: a distributed, non-model-invocable runner (/benchmark-skill
  <name>) that orchestrates force-invoked new-skill and skill-absent baseline
  arms plus a blind per-trial judge as in-session subagents.
- Deterministic core (Python): a pure transform over the collected run data
  that sums per-arm usage, collapses each trial to WIN/TIE/LOSS, applies the
  efficacy pass rule (wins >= 3, losses <= 1), and renders a self-contained
  report. Parameterized over arms and comparisons; two-arm here.
- Fixture-driven no-LLM check (checks/benchmark-core.nix) wired into flake.nix.
- Real tests tree for axi-review; a live run produced its efficacy report.

flake.nix

@@ -84,6 +84,13 @@
             shell-hook = import ./checks/shell-hook.nix {
               inherit pkgs mkSkill mkSkillsShellHook;
             };
+
+            # Proves the benchmark skill's deterministic core at its single seam:
+            # it feeds committed fixtures to the core and asserts the metrics, the
+            # Efficacy verdict, and the rendered report.
+            benchmark-core = import ./checks/benchmark-core.nix {
+              inherit pkgs;
+            };
           };
         }
       );