sandbox: add per-environment host workspaces

[Saiph77]

Summary

• add a host sandbox backend with per-environment workspace, home, and temp directories
• retain the existing single-directory HOST_WORKSPACE_ROOT behavior while adding HOST_WORKSPACES_ROOT
• serialize commands within one environment while allowing separate environments to execute independently
• preserve read-only workspace layers, background processes, and sandbox home migration

Configuration

SANDBOX_BACKEND=host
HOST_WORKSPACES_ROOT=/srv/qm/workspaces

HOST_WORKSPACE_ROOT remains supported for existing trusted single-workspace installations. Configuring both roots is rejected.

This backend provides directory routing for trusted users. It does not provide filesystem, process, network, CPU, or memory isolation from the host.

Validation

• focused Host/config/migration tests: 40 passed
• npm run typecheck
• npm run lint
• git diff --check
• independent security and migration review

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}