feat: external compute driver extension point

Description

Track the extensibility capability proposed by NVIDIA/OpenShell#1703: allowing the gateway to connect to an operator-owned, out-of-tree compute driver over a Unix domain socket via --compute-driver-socket / OPENSHELL_COMPUTE_DRIVER_SOCKET.

At a high level, this lets OpenShell keep the gateway and sandbox lifecycle contract stable while allowing compute backends to live outside the core repository and speak the existing compute_driver.proto interface.

Context

• Implementation PR: NVIDIA/OpenShell#1703
• Parent extensibility roadmap: NVIDIA/OpenShell#1904

What This Unlocks

• Lets operators integrate platform-specific compute backends without requiring every backend to be built into OpenShell.
• Allows managed or opinionated infrastructure environments to provide their own sandbox lifecycle driver while using the upstream gateway image.
• Keeps the in-tree driver set focused on common backends while enabling external driver experimentation and adoption.
• Creates a path for partner, internal, or deployment-specific compute drivers to evolve independently from the core gateway release cycle.

Definition of Done

• The external compute driver activation path is accepted, merged, or closed with a clear disposition.
• Follow-on issues are created for any accepted v1 hardening, documentation, registry, or testing work.
• Child work is linked under the relevant roadmap/project item so project tracking stays accurate.