Validation: isolate submitted code in author Sandboxes

[zkwentz]

What & Why

The first remote runner allowed submitted build/runtime processes too much proximity to the trusted report path. This hardening layer separates subject execution by UID, keeps trusted inputs read-only, and makes teardown explicit.

Risk & Review Guidance

• Risk level: HIGH — fixes an isolation boundary around untrusted code
• Task class: security
• Review focus: src/openenv/validation/remote.py:242 for UID/report-path setup; src/openenv/validation/local.py for subject UID/GID propagation; remote/local isolation tests
• Blast radius: Without this boundary, submitted code could forge or mutate the author report or survive beyond validation.

Decisions & Alternatives

• Root-owned report writer, unprivileged subject UID — rejected same-UID convenience.
• Read-only source and validator trees — subject code gets only the access required to install/run.
• Kill then close in finally — teardown is mandatory on success and failure.

Verification

• PASS: PYTHONPATH=src:envs .venv/bin/python -m pytest tests/ -q -m 'not integration' — 1,559 passed, 114 skipped, 31 external integration tests deselected on the complete stack.
• PASS: Fork whole-stack CI — 7/7 checks passed, including Python 3.11/3.12, lint, docs, lock validation, and package smoke tests.
• NOT verified: Kernel-level containment is delegated to the dedicated HF Sandbox platform and was not live-tested here.

Scope & Non-Goals

• Task-provided verifier isolation remains advisory in the author runner until a separate verifier Sandbox exists.

Stack

PR 13 of 17; depends on #953.

Full 17-PR stack

1. #942 RFC 008: Spec-neutral local and remote validation architecture
2. #943 Validation foundation: spec-neutral contracts and adapters
3. #944 Validation policy: spec-aware check registry and planner
4. #945 Validation executor: hardened shared reports and eligibility
5. #946 Local validation API: execution-model-aware profiles
6. #947 CLI: validation profiles and shared reports
7. #948 HF runtime: dedicated sandbox execution mode
8. #949 HF certification: enforce trusted dedicated isolation
9. #950 [RFC 008] Define remote author validation and publish gating
10. #951 Validation: unify CLI reports and add a strict publish profile
11. #952 Validation: add structured author diagnostics and remediation
12. #953 Validation: run author validation in dedicated HF Sandboxes
13. #954 Validation: isolate submitted code in author Sandboxes ← current
14. #955 Validation: bind author reports to policy and source
15. #956 CLI: scaffold publish requirements
16. #957 CLI: gate pushes on staged remote validation
17. #958 Docs: explain local, remote, and publish validation

All upstream PRs remain draft while RFC 008 is under review.