Validation: run author validation in dedicated HF Sandboxes

[zkwentz]

What & Why

Authors need an isolated way to run the same strict plan before publishing untrusted or dependency-heavy environments. This PR adds openenv validate --remote, using a fresh dedicated HF Sandbox and returning the shared report.

Risk & Review Guidance

• Risk level: HIGH — uploads source and executes submitted build/runtime code remotely
• Task class: security
• Review focus: src/openenv/validation/remote.py:242 for the Sandbox command; src/openenv/validation/remote.py:588 for lifecycle/report handling; src/openenv/cli/commands/validate.py for remote CLI dispatch
• Blast radius: A flaw can leak credentials, trust a forged report, leave remote compute running, or validate different source than requested.

Decisions & Alternatives

• Never use SandboxPool — uploaded environments are cross-user trust boundaries.
• Upload validator source with the revision — local and remote use the initiating implementation, not an ambient package guess.
• Never forward the HF token — only the control plane receives credentials.

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: No live HF Sandbox smoke test was run because this machine has no HF token.

Scope & Non-Goals

• This is unofficial author validation, not the webhook coordinator or certification registry.

Stack

PR 12 of 17; depends on #952.

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 ← current
13. #954 Validation: isolate submitted code in author Sandboxes
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.