feat(sandbox): add structured direct execution

[Nzoa-app]

Summary

• add an additive structured runDirect contract with executable, argv, stdin, cwd, explicit environment, timeout, output limits, and cancellation
• implement native no-shell execution for local, AWS microVM, and Sprites; unsupported backends fail closed
• enforce descriptor-owned environment allowlists, fixed runtime PATH, process-group termination, bounded output, and strict output masking
• quarantine resident credentials for all nonlegacy broker modes and remove insecure shell fallback
• preserve the legacy shell execution route for legacy tools

Security properties

• no sh -c, bash -c, shell=true, eval, string command parsing, globbing, redirection, or automatic shell fallback inside runDirect
• broker errors and direct-execution errors are generic
• direct-only services are not advertised on unsupported backends
• remote helpers independently validate environment keys and kill the full process group on abort/timeout/output overflow

Validation

• Node 24 typecheck
• ESLint
• Prettier check
• Oxlint
• 44 focused tests passing
• gitleaks diff scan clean
• targeted Semgrep clean
• independent security review: no High/Medium findings

This is the upstream prerequisite for the Nzoa Employee incident remediation. It contains no credential values and does not start or restart any soak window.

[Nzoa-app]

@16francej Maintainer review/merge authority is required for this upstream security boundary.

Updated exact head: c0cbe06.

The latest surgical commit closes the cancellation gap discovered during the downstream cloud gate: caller abort now identifies the exact active Sprites exec session with a per-request UUID, requires authenticated fail-closed kill-stream completion, cleans up all post-dispatch transport/protocol failures, and terminates the supervised process tree even while structured stdin is still open.

Evidence on this exact head:

• independent security review: PASS;
• independent Sprites API/contract/concurrency review: PASS;
• full Node 24 suite: 4,209 total / 4,045 pass / 0 fail / 164 declared skips;
• focused direct-execution suite: 19 pass / 0 fail / 1 Linux-only skip;
• TypeScript, ESLint, Prettier, git diff check, and gitleaks-on-diff: PASS.

Upstream main has since advanced by four commits to 090f490. A read-only merge-tree simulation is conflict-free, producing tree c93ff2eed0416a945f932bb3417576860c664960; focused tests on that combined tree passed 67 runnable / 0 failed / 1 Linux-only skip, and TypeScript passed.

Downstream Nzoa propagation, cloud deployment, Cloud Independence Gate, and replacement soak T0 remain blocked until this PR receives upstream acceptance.