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.

.claude/tasks/0029-bench-run-loop-cli.md

@@ -11,7 +11,34 @@ Each cell defaults to five trials with a reporting floor of three. Because resul
 
 ## Acceptance criteria
 
-- [ ] The command runs a single selected `(arm, task)` cell on demand.
-- [ ] A cell defaults to five trials, and the reporting floor of three is respected.
-- [ ] Re-running an already-sampled cell appends new trials rather than overwriting prior samples.
-- [ ] The command drives the runner and store built in earlier slices rather than reimplementing orchestration.
+- [x] The command runs a single selected `(arm, task)` cell on demand.
+- [x] A cell defaults to five trials, and the reporting floor of three is respected.
+- [x] Re-running an already-sampled cell appends new trials rather than overwriting prior samples.
+- [x] The command drives the runner and store built in earlier slices rather than reimplementing orchestration.
+
+## Implementation Notes
+
+**Two seams, matching the harness's established split.**
+The pure orchestration is `runCells` in `bench/run-loop.ts`: it decides only how many trials to run and at what trial numbers, then delegates provision/run/score/append to `runCell` and the sample store — it reimplements no orchestration (criterion 4).
+The maintainer command is `bench/run.ts`: `parseRunArgs` is the pure, unit-tested argument seam, and `runBenchCommand`/`main` are the live boundary (resolve credentials, probe self-review, select the task, drive `runCells`).
+Following `bench/README.md`'s convention, the live boundary is validated by running it rather than by mocked unit tests — and, deliberately, by reusing pieces already smoke-covered (`runCell` via the runner smoke, `detectSelfReviewSupport` via the self-review smoke, `liveBenchHost` via the seed smoke) rather than adding a new smoke test that would spend real tokens on every invocation.
+
+**Deepening by highest trial number, not sample count.**
+`runCells` continues numbering from `max(existing trial) + 1`, not from the sample count, so an earlier invalid attempt (which records no sample and leaves a gap) can never cause a later sitting to reuse a trial number.
+
+**TDD sequencing deviation.**
+The run loop is a small cohesive unit, so its first GREEN already carried the trial-numbering and floor logic; tests 2 and 3 (deepening, invalid/floor) and the parser override/reject/help tests are therefore passing characterization/regression tests rather than red-first cycles.
+Each was still written test-first by the test-writer sub-agent, from the public interface only, with independent expected literals — they remain discriminating guards.
+
+**Running the harness's TypeScript.**
+Node's native type stripping does not rewrite `.js` import specifiers to `.ts`, which the whole `bench/` tree relies on, so the command runs under `tsx` (added as a devDependency) via `npm run bench:run`.
+The Claude Agent SDK is now formally declared — as an *optional* `peerDependency` (`@anthropic-ai/claude-agent-sdk`) — so it is documented but neither installed for package consumers nor pulled into CI's `npm ci`; the maintainer installs it for live runs, matching how the driver already treats it as an optional peer.
+The default store root `bench/results/` is gitignored.
+
+**Review finding addressed — `--model` dropped.**
+The spec fixes a single model across all arms so the comparison measures the tool, not the model.
+A per-cell `--model` override (flagged as scope creep by the spec-fidelity review) would let arms drift onto different models, so it was removed.
+`--turn-cap` and `--wall-clock-ms` were kept: they are safe-defaulted bounds a maintainer may legitimately need to raise for a heavier task, and they do not affect cross-arm comparability.
+
+**Deferred to the aggregator (slice 0030).**
+The CLI tally reports recorded/invalid counts and reporting-floor status, but does not break failures out by tag (incorrect/confused/hung); those tags are on every stored sample and are the reporting slice's job to surface.