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-lock.json

@@ -19,11 +19,20 @@
       "devDependencies": {
         "@types/node": "^20.19.0",
         "@vitest/coverage-v8": "^3.2.7",
+        "tsx": "^4.23.1",
         "typescript": "^5.8.0",
         "vitest": "^3.2.0"
       },
       "engines": {
         "node": ">=20"
+      },
+      "peerDependencies": {
+        "@anthropic-ai/claude-agent-sdk": ">=0.3.0"
+      },
+      "peerDependenciesMeta": {
+        "@anthropic-ai/claude-agent-sdk": {
+          "optional": true
+        }
       }
     },
     "node_modules/@ampproject/remapping": {
@@ -2229,6 +2238,25 @@
         "node": ">=14.0.0"
       }
     },
+    "node_modules/tsx": {
+      "version": "4.23.1",
+      "resolved": "https://registry.npmjs.org/tsx/-/tsx-4.23.1.tgz",
+      "integrity": "sha512-GQHnkIfxyx1wYCOS/wonik5MVRZU9hi1TEZmzGZSCJB1y9YgoZ8H6itNE/u4suE+yLmOzuE4E5S4TZ/ZX2wcWQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "esbuild": "~0.28.0"
+      },
+      "bin": {
+        "tsx": "dist/cli.mjs"
+      },
+      "engines": {
+        "node": ">=18.0.0"
+      },
+      "optionalDependencies": {
+        "fsevents": "~2.3.3"
+      }
+    },
     "node_modules/typescript": {
       "version": "5.9.3",
       "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.9.3.tgz",