Rebuild @repo/sandbox-execution into a byte-first, argv-only substrate for supervised processes and PTYs, replacing the persistent command-session machinery it used to carry.

The old substrate tied execution to a long-lived shell, multiplexed output as interleaved text/base64, and kept a second, parallel PTY implementation. That coupling is what made recovery, cancellation, and output framing hard to reason about upstream. This replaces it with one supervisor that launches argv processes and owns the process tree (including reaping descendants), and one log engine — a sequenced byte log with bounded retention and cursor-based replay/follow — shared by both processes and the canonical PTY primitive. Output stays raw bytes end to end; decoding is a consumption-time concern for higher layers.

The package deliberately depends on nothing from @repo/shared, capnweb, Cloudflare/Durable Object types, or SDK ID and readiness policy. It is the pure Linux/Bun execution layer; the container and SDK layers adapt it.

The package itself typechecks and its Docker-backed tests pass. Repository-wide consumers are migrated in the following PRs, so the full monorepo is not expected to typecheck at this intermediate stack point. The process-group tests rely on the tini reaper in the sandbox images, and a negative control proves the redirected-descendant test genuinely fails without it.

Part of the unified process-execution stack.

Core stack — each builds on the previous:

•  5  Replace sessions with recoverable processes #819 — Replace sessions with recoverable processes
•  4  Rebuild container runtime around process RPC #818 — Rebuild container runtime around process RPC
•  3  Define shared process contracts and errors #817 — Define shared process contracts and errors
•  2  Build execution substrate for supervised processes #816 — Build execution substrate for supervised processes  👈
•  1  Guard pull request quality configuration #815 — Guard pull request quality configuration
• next

Migrations — each builds on  5  #819:

• Migrate bridge clients to process resources #820 — Migrate bridge clients to process resources
• Migrate examples to unified execution #821 — Migrate examples to unified execution
• Migrate E2E workflows to process handles #822 — Migrate E2E workflows to process handles
• Document unified process execution #823 — Document unified process execution
• Run execution lifecycle checks in CI #824 — Run execution lifecycle checks in CI