feat: add benchmark single-cell runner (task 0027)

Thread every benchmark layer to run one (arm, task, trial) cell end to
end: provision and seed a throwaway repository, run the agent under the
active arm bounded by a turn cap and a wall-clock backstop, audit the
transcript, capture and score the post-run state, append the sample, and
delete the repository.

- runner.ts: runCell orchestration behind the BenchHost and AgentDriver
  seams, so the flow is unit-tested with fakes while the live wiring is
  validated by a smoke run; turn-cap and wall-clock failures are tagged
  confused-versus-hung, and a leaked transcript is flagged invalid.
- audit.ts: the post-run transcript audit plus the shared
  foreignToolReason predicate both isolation enforcement points consume.
- task.ts: the runnable BenchTask wrapper and one sample single-mutation
  task exercising the full path.
- snapshot.ts: captureRepoState, the seed's read-back counterpart, in the
  RepoState shape the checker diffs against.
- host.ts / sdk-driver.ts: the live BenchHost and the Claude Agent SDK
  driver (an optional peer, loaded via dynamic import) for real runs.
- runner.smoke.test.ts: the live tracer-bullet tier, skipping cleanly
  when no host or SDK is configured.

bench/seed.ts

@@ -73,8 +73,12 @@ async function requireOk(res: Response, method: string, path: string): Promise<R
   return res;
 }
 
-/** Issue a request and require a 2xx, returning the parsed JSON body. */
-async function send<T>(
+/**
+ * Issue a request and require a 2xx, returning the parsed JSON body. Exported so
+ * the post-run snapshot capture (snapshot.ts) reads the live repository through
+ * the same authenticated round-trip the seed writes through.
+ */
+export async function send<T>(
   access: BenchAccess,
   method: string,
   path: string,