fix(sandbox): disable runtime when KVM is unavailable

[btalx]

Context

On Linux hosts without accessible /dev/kvm, Boxlite can abort the backend process when an agent invokes execute_sandboxed_code. This surfaced in Nao chats using Claude Opus 5 as a network error after the response began.

Linear: ENG-9189

Changes

• Check effective-user read/write access to /dev/kvm before importing Boxlite on Linux
• Disable sandbox tool exposure and sandbox-dependent skill guidance when the host is unsupported
• Preserve the existing native runtime path on non-Linux hosts
• Generalize the experimental-settings unavailable message
• Add regression coverage for missing, denied, and usable KVM access

Verification

• Linux container without /dev/kvm: backend import succeeds, isSandboxAvailable is false, and sandboxRuntime is null
• Impacted tests: 39 passed
• Formatting, TypeScript, and ESLint passed