HF certification: enforce trusted dedicated isolation

[zkwentz]

What & Why

A caller must not be able to label loosely configured HF execution as official certification. This PR locks dedicated provider settings and verifies isolation evidence before certification can proceed.

Risk & Review Guidance

• Risk level: HIGH — this is the official-certification trust gate
• Task class: security
• Review focus: src/openenv/validation/security.py:14 for official provider checks; certification additions in src/openenv/core/containers/runtime/hf_sandbox_provider.py; tests/test_validation/test_security.py for credential and host-isolation cases
• Blast radius: A bypass could certify pooled execution, forwarded credentials, mutable evidence, or unverified host isolation.

Decisions & Alternatives

• Reject rather than downgrade official runs — certification fails closed when required isolation evidence is absent.
• Lock provider configuration — rejected trusting caller-supplied labels such as official=True.
• Scan credential-like material broadly — covers tokens, DSNs, signed URLs, and private keys.

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 production coordinator or multi-host certification run exists yet.

Scope & Non-Goals

• This does not deploy the Hub coordinator or signed report registry.

Stack

PR 8 of 17; depends on #948.

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 ← current
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
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.