Make WP Codebox a thin Homeboy execution target

Related: Extra-Chill/homeboy#8282, Extra-Chill/homeboy#7530, Extra-Chill/homeboy-extensions#2154

## Problem

WP Codebox GitHub agent-task execution and Homeboy Extensions both own task translation, workspace snapshotting, patch application, verification, publication, result projection, and terminal status. This creates two orchestration authorities and extensive path/artifact compatibility handling.

## Desired outcome

Extend the existing Homeboy runner workload/execution envelope and WP Codebox runtime contracts so Codebox owns WordPress sandbox materialization, execution, and native artifact emission only. Homeboy owns lifecycle, retries, application, verification policy, promotion, publication, and final projection. Extensions becomes a thin provider selector/adapter.

## Acceptance criteria

- One typed request enters Codebox and one canonical typed result returns.
- Codebox emits typed artifacts and workspace delta without filename inference or host-path leakage.
- Homeboy applies, verifies, promotes, and publishes exactly once.
- GitHub workflow-side apply/publication/result projection is removed after migration.
- End-to-end tests cover success, no-op, malformed artifacts, verification failure, retry, and expired source materialization.

## AI assistance
- **AI assistance:** Yes
- **Tool(s):** OpenCode with openai/gpt-5.6-sol
- **Used for:** Traced the WP Codebox and Homeboy Extensions agent-task execution paths and drafted the ownership consolidation.

Start by tracing the WP Codebox and Homeboy Extensions agent-task execution paths referenced in the related issues, then review the existing Homeboy runner workload/execution envelope and WP Codebox runtime contracts. Done means one typed request and result, typed artifacts without host-path leakage, centralized lifecycle ownership, and end-to-end coverage for success, no-op, malformed artifacts, verification failure, retry, and expired source materialization.

Written by the indexing model from the issue text.