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.

tests/skills/axi-review/basic-cli-review/fixture/greet

@@ -0,0 +1,41 @@
+#!/usr/bin/env sh
+# greet: a tiny CLI, deliberately imperfect, for exercising an AXI review.
+
+usage() {
+  cat <<'EOF'
+greet — say hello
+
+USAGE:
+  greet hello <name>     print a greeting
+  greet json <name>      print the greeting as JSON
+  greet --help           show this help
+EOF
+}
+
+case "$1" in
+  --help|-h|help)
+    usage
+    ;;
+  hello)
+    if [ -z "$2" ]; then
+      echo "error: missing name" >&2
+      exit 1
+    fi
+    echo "Hello, $2!"
+    ;;
+  json)
+    if [ -z "$2" ]; then
+      echo "error: missing name" >&2
+      exit 1
+    fi
+    printf '{"greeting":"Hello","name":"%s"}\n' "$2"
+    ;;
+  "")
+    usage >&2
+    exit 2
+    ;;
+  *)
+    echo "error: unknown command '$1'" >&2
+    exit 2
+    ;;
+esac