Add Agent Runtime Fabric MVP: control plane, worker, policy, and Dockerfile

[ethgr0wth]

Motivation

• Provide a secure, disposable, policy-restricted runtime fabric scaffold that separates control, persistence, and low-trust execution planes.
• Deliver a runnable MVP execution substrate that enforces tool mediation and scoped package installs for Python and Node.js workloads.
• Supply operator-friendly artifacts (policy, Dockerfile, and docs) to make local deployment and iterative hardening straightforward.

Description

• Add a new agent-runtime-fabric/ module with a high-level design README.md and mvp-policy.yaml runtime profile that defines resources, filesystem zones, network allowlists, and tool policies.
• Implement a FastAPI control plane in runtime_fabric_mvp/control_plane.py exposing session creation, run_code, install_package, export_artifact, and session reset endpoints.
• Add runtime_fabric_mvp/runtime_worker.py, policy.py, and schemas.py to manage session-scoped workspaces, enforce runtime policies, execute Python/Node code, perform pip/npm installs into session-local targets, and export artifacts to an output store.
• Include runtime_fabric_mvp/README.md, requirements.txt, and a Dockerfile to build and run the MVP service with uvicorn.

Testing

• No automated tests were added or executed as part of this change.

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Codex Task