feat: add benchmark run-loop command (task 0029)

Add the maintainer-facing command that runs one chosen benchmark cell on
demand, so only the token budget available at that moment is spent.

runCells (bench/run-loop.ts) runs one (arm, task) cell for a batch of
trials — defaulting to five with a reporting floor of three — by driving
the existing single-cell runner and the append-only sample store rather
than reimplementing orchestration. Re-running a cell deepens it: trial
numbering continues past the highest trial the cell already holds and the
new samples append, so a cell's sample size grows across sittings without
overwriting prior runs.

bench/run.ts is the command: parseRunArgs is the pure, unit-tested
argument seam, and runBenchCommand is the live boundary that resolves host
access, resolves the scored suite against the host's self-review support,
selects the task, and drives the run loop. It is invoked via the new
bench:run npm script, run under tsx (a new devDependency) because the
harness's .js-specifier imports need a TypeScript-aware runner. The Claude
Agent SDK is now declared as an optional peerDependency — documented but
neither installed for package consumers nor pulled into CI.

Every arm runs on the driver's single fixed model; the command exposes no
per-cell model override that could break cross-arm comparability. The
default store root bench/results/ is gitignored.

package.json

@@ -36,7 +36,8 @@
     "test:e2e": "vitest run --config vitest.e2e.config.ts",
     "test:pack": "vitest run --config vitest.packaging.config.ts",
     "test:bench": "vitest run --config vitest.bench.config.ts",
-    "test:bench:smoke": "vitest run --config vitest.bench-smoke.config.ts"
+    "test:bench:smoke": "vitest run --config vitest.bench-smoke.config.ts",
+    "bench:run": "tsx bench/run.ts"
   },
   "dependencies": {
     "@toon-format/toon": "^2.3.0",
@@ -46,7 +47,16 @@
   "devDependencies": {
     "@types/node": "^20.19.0",
     "@vitest/coverage-v8": "^3.2.7",
+    "tsx": "^4.23.1",
     "typescript": "^5.8.0",
     "vitest": "^3.2.0"
+  },
+  "peerDependencies": {
+    "@anthropic-ai/claude-agent-sdk": ">=0.3.0"
+  },
+  "peerDependenciesMeta": {
+    "@anthropic-ai/claude-agent-sdk": {
+      "optional": true
+    }
   }
 }