HF runtime: dedicated sandbox execution mode

[zkwentz]

What & Why

Official and author-isolated validation require a dedicated HF Sandbox execution mode rather than a same-user pool. This PR adds dedicated lifecycle, flavor, timeout, token-forwarding, and file-transfer support to HFSandboxProvider.

Risk & Review Guidance

• Risk level: HIGH — remote execution, credentials, and lifecycle cleanup
• Task class: security
• Review focus: src/openenv/core/containers/runtime/hf_sandbox_provider.py:320 for provider mode selection and teardown; tests/test_core/test_hf_sandbox_provider.py for dedicated-vs-pooled behavior
• Blast radius: A mistake can forward credentials, reuse an unsafe trust domain, leak a Sandbox, or run on unintended hardware.

Decisions & Alternatives

• Dedicated mode is explicit — pools remain available only for same-user trusted workloads.
• Token forwarding defaults off for isolated work — the control-plane token should not enter subject processes.
• Provider owns teardown — rejected relying on caller garbage collection.

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 provider mode alone does not make a run official; policy locking is added in the next PR.

Stack

PR 7 of 17; depends on #947.

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 ← current
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
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.