Sandboxd local runtime — scoped provider preview 2

Experimental matched Linux engine/runtime bundles, source 9b1a44e. Native ARM64 and x64 Go tests, race checks, image builds and real sandbox lifecycle passed in CI run 38807367363. Adds opt-in app-scoped model access and private delivery to the runtime supervisor; scoped mode requires authenticated management and remains OFF by default. No automatic production update. Claude Code and Pi are not installed; bundled OpenCode/Codex authentication requires separate validation. The Desktop OpenClaw install/browser flow has Linux ARM64 development evidence with synthetic credentials, not real model inference or native Windows/macOS proof. Automatic installed-agent credential renewal, reboot redelivery and native Desktop validation remain unfinished. Archives contain immutable untagged image IDs and must be independently hash-pinned. This is a development runtime dependency, not a Windows installer or complete consumer release. Build evidence: https://github.com/tastyeffectco/sandboxd/actions/runs/38807367363