refactor(benchsdk): split SDK into @benchsdk/api, @benchsdk/worker, and @benchsdk/client umbrella

[dtice25]

Summary

This PR begins the benchsdk v2 migration by splitting the SDK into focused, single-purpose packages:

• @benchsdk/api — typed REST client (createBenchmarkClient) and shared platform data types. No worker runtime.
• @benchsdk/worker — the runWorker engine, BenchmarkReporter, createSystemMetricsCollector, and participant helpers. Depends on @benchsdk/api.
• @benchsdk/client — remains a backwards-compatible umbrella package. It re-exports from @benchsdk/api and @benchsdk/worker, and its createBenchmarkClient returns a client that still exposes runWorker(options) so existing consumers and tests compile unchanged.

What changed

// @benchsdk/api — thin REST + types only
export { createBenchmarkClient, BenchmarkApiError } from '@benchsdk/api';

// @benchsdk/worker — worker runtime only
export { runWorker, BenchmarkReporter, claimBenchmarkReporter,
        createSystemMetricsCollector, filterParticipantsByEnv, selectParticipants } from '@benchsdk/worker';

// @benchsdk/client — backwards-compatible umbrella
import { createBenchmarkClient } from '@benchsdk/client';
const client = createBenchmarkClient({ apiKey });
await client.runWorker({ ... }); // still works

• Extracted packages/benchsdk/src/client.ts into packages/benchsdk-api/src/client.ts with the REST methods only.
• Extracted the runWorker implementation into packages/benchsdk-worker/src/worker.ts as a standalone runWorker(client, options) function.
• Copied reporter.ts, metrics.ts, and participants.ts into @benchsdk/worker.
• packages/benchsdk/src/client.ts is now a thin wrapper that creates an @benchsdk/api client and attaches runWorker.
• packages/benchsdk/src/reporter.ts, metrics.ts, and participants.ts are now re-export shims from @benchsdk/worker.
• Added the two new packages to pnpm-workspace.yaml and their dependencies to pnpm-lock.yaml.
• Updated packages/benchsdk/src/__tests__/public-api.contract.test.ts so VAL-SDK-090 parses the new .d.ts barrel shape, which now contains both local type declarations and re-exports from workspace packages.

Verification

• pnpm -r --filter "./packages/**" build passes for @benchsdk/api, @benchsdk/worker, @benchsdk/client, and @benchsdk/runner.
• pnpm typecheck passes.
• pnpm --filter @benchsdk/client test passes (119 passed, 1 skipped).
• pnpm --filter @benchsdk/runner test passes (86 passed).
• pnpm -r --filter "./packages/benchsdk" --filter "./packages/benchsdk-api" --filter "./packages/benchsdk-worker" lint passes (runner lint is missing eslint in its dev deps, pre-existing).

What comes next

• Migrate @benchsdk/runner to import from @benchsdk/api and @benchsdk/worker directly instead of @benchsdk/client.
• Move the display/scoring manifest types into @benchsdk/api so both benchmarks and benchmarks-platform share one source of truth.
• Add typed metric sinks (metrics.timing(...) / metrics.count(...)) and platform-side scoring.

This is a foundation-only PR; no benchmark schema or runner behavior is changed yet.

Link to Devin session: https://app.devin.ai/sessions/e5c1507360b24f6f89327ccc975ae961
Requested by: @dtice25

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