#9452 improvement(sandbox): pause idle chat workbenches instead of keeping them running

Summary

• Chat workbench sandboxes pause after a 5-minute idle lease instead of running for 20 minutes and then being killed. Resume keeps files, packages and processes and takes well under a second. Paused sandboxes aren't billed and don't count toward the provider's concurrency limit
• New cleanup-session-sandboxes cron (every 5 min) kills workbenches paused for more than 15 min, so retention stays where it was: a workbench is discarded about 20 min after its last use
• Each kill runs under the session lock and re-checks the sandbox's own getInfo, because the provider's list trails resumes and kills by a few seconds. autoResume: false, so only a locked reconnect can wake a paused sandbox
• Streamed workbench writes and upload snapshots now lease their full 10-min transfer budget, so a machine can't pause mid-transfer under the shorter idle lease
• E2B Mothership sandboxes are priced at their template's real 2 vCPU / 1 GiB instead of the 4 GiB Function profile (sandbox time was overcharged by ~41%)
• Cron registered in helm, docker crontab, the self-hosting docs and the API-validation allowlist. The hosted schedule needs the matching infra cron entry (separate PR)

Replaying a day of real sandbox activity through the new policy: ~38% fewer running sandbox-hours, ~23% lower peak concurrency, and idle gaps of 5–20 min resume the same machine instead of keeping it running.

Type of Change

• Improvement

Testing

• New opt-in live E2E session-pause.live.test.ts against real E2B, with a real mothership-shell sandbox: auto-pause on lease expiry; reconnect resumes with a file intact; kill is refused for a too-recent pause and for a running sandbox; kill succeeds once expired. Red-checked: with kill-on-timeout instead of pause, it fails
• New pricing row for the Mothership profile and the write-lease assertion: both fail with their fix reverted
• Targeted vitest across lib/execution/remote-sandbox, lib/billing and lib/mothership, all passing
• Biome on changed files; check:api-validation:strict, check:unused-exports, check:comment-hygiene, check:source-conventions pass locally. Type-check and the full audit gate run in CI

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)