Optimize Beam JS sandbox lifecycle and command startup

[luke-lombardi]

What changed

• Add the sandbox wait exec option and consume inline exit status/stdout/stderr responses.
• Avoid follow-up status and output requests when a short blocking command completes inline.
• Singleflight shared runtime preparation so concurrent sandbox creates do not race image/file synchronization.
• Add concurrency and inline-result regression coverage.
• Bump the SDK version to 1.0.13.
• Preserve the sandbox parity, examples, and e2e coverage already present on this branch.

Why

ComputeSDK's node -v probe paid several avoidable HTTP round trips and a 100 ms polling interval after every sandbox became ready. Concurrent first-use creates could also collide in file synchronization and fail with Sync already in progress.

Impact

With the coordinated beta9 polling change, the latest-upstream ComputeSDK production-order run completed:

• sequential: 100/100, median 169.5 ms, reported p99 214.9 ms
• staggered: 100/100, median 156.5 ms, reported p99 228.8 ms

Validation

• npm test -- --runInBand — 140 passed, 3 skipped
• npm run build — CJS and ESM builds clean
• npm pack --dry-run — package validated as 1.0.13
• Latest ComputeSDK benchmark upstream with the published 1.0.12 package patched only with this candidate

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Summary by cubic

Improves sandbox startup and stability with inline exec results, transient retries, and shared runtime preparation. Also adds Docker support, filesystem helpers, sandbox examples, and opt-in e2e tests; workspaceId is now optional.

• New Features

  • Exec supports wait to return inline exitCode/stdout/stderr for short commands; added execShell, status/poll/wait, and server-backed listProcesses.
  • Filesystem helpers: writeText/writeBytes, readText/readBytes, and remove.
  • Docker-in-Docker: Image.withDocker() and SandboxDockerManager (docker.version, docker.run, docker.compose); ensures daemon readiness and uses host network/pid namespaces.
  • Added sandbox examples and production e2e tests (Docker tests are opt-in).

• Performance

  • Avoids follow-up status/output requests for short blocking commands via inline wait; reduces polling and round trips.
  • Singleflight runtime preparation to prevent first-use sync races; added transient retry for status/stdout/stderr calls.

^{Written for commit adcf058. Summary will update on new commits.}

[luke-lombardi]

Superseded by beam-cloud/beam-client#74. The publishable JavaScript SDK source now lives under beam-client/js; the replacement PR contains only the canonical source and tests.