Implement SBX read-only host-config projection at the stable guest path #606

Problem

ADR 0035 specifies a per-sandbox host-config directory mounted read-only at /var/lib/acq/host on both backends. MSB implements this with --volume <hostdir>:/var/lib/acq/host:ro, but the current SBX adapter does not mount host config at all. SBX positional :ro mounts preserve the host absolute path in the guest and do not provide a distinct destination path.

This blocks #600: released provider bundles can be validated and staged under host config, but cannot be presented at the stable provider-contract path /var/lib/acq/host/models/providers/<providerId> with a proven VMM-enforced read-only boundary. Guest copies or ordinary kit overlays remain writable by the passwordless-sudo agent and are not acceptable substitutes.

Acceptance criteria

• A supported SBX mechanism mounts one sandbox's host-config directory read-only at /var/lib/acq/host, or the architecture adopts an equally stable provider-neutral path with explicit consumer wiring.
• Guest root cannot modify staged files.
• Host and guest paths, lifecycle cleanup, and sandbox-name collision handling are tested offline.
• Live SBX validation records the actual mount and write-denial result.
• Project selected provider inputs read-only for agent kits #600 can stage provider inputs once and consume the same guest contract on SBX and MSB.

Relationships

Blocks: #600.
Related: ADR 0035 and GSA-TTS/agentic-coding-patterns#357.

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