docs(adr): propose ADR-0008 pluggable execution environments

[lyarwood]

Summary

• Proposes extending the operator to support alternative execution environments beyond Kubernetes Pods
• Introduces an ExecutionEnvironment interface with KubeVirt VM as the first non-pod implementation
• Motivated by e2e testing workflows (KubeVirt, kubernetes-mcp-server) that require nested containers or full OS environments, which are impossible in the current capability-dropped, non-root runner pods

Scope of this ADR

This ADR covers the full picture: the ExecutionEnvironment abstraction, registry schema changes (executionKind + vmConfig), operator dispatch, KubeVirt VM lifecycle, cloud-init bootstrap, and networking. I'm happy to break it out into several smaller ADRs if that's easier to review, for example:

1. ADR-0008a: Extract ExecutionEnvironment interface from the monolithic pod builder (pure refactor, no new functionality)
2. ADR-0008b: KubeVirt VM execution environment implementation (depends on 0008a)

The core idea is that the upper layers (PlatformBridge, backend AG-UI proxy, frontend) are already execution-agnostic — the work is isolated to the operator and the agent registry.

Context

See kubevirt-ai-helpers: Future Work — In-Session Eval Testing for the motivating use case.

Feedback welcome

This is a suggestion from an external contributor — I'd appreciate any feedback on:

• Whether this direction aligns with the platform's roadmap
• Whether the ExecutionEnvironment interface abstraction makes sense at the operator level
• Whether executionKind on AgentRuntimeSpec is the right place for runtime selection vs a CRD-level field
• Any concerns about KubeVirt as a hard/optional dependency

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