harness: add OpenHands CLI/headless support

[willwashburn]

Summary

Add OpenHands CLI/headless execution as a Relay-supported coding harness.

Context

OpenHands has a command-line execution surface and can run coding tasks locally. This may fit Relay better as a headless/process integration than as an interactive PTY agent.

Sources:

• https://docs.openhands.dev/openhands/usage/cli/command-reference
• https://github.com/All-Hands-AI/OpenHands

Scope

• Evaluate the best integration mode: process, headless provider, or workflow-only harness.
• Add CLI metadata and command builder support if suitable.
• Add output/completion collector behavior.
• Investigate whether Relaycast messaging can be injected or whether this is batch-only.
• Add docs and smoke tests.

Acceptance

• OpenHands has a documented Relay integration path or a clear no-go rationale.
• If supported, workflow execution can run a task and collect output deterministically.
• Docs state sandbox/install requirements and limitations.