Expose parent agent task runner contract

[chubes4]

Summary

• Adds a parent_request adapter for homeboy/wp-codebox-task-request/v1 so external executors can call wp-codebox/run-agent-task without synthesizing full WP Codebox recipes.
• Passes provider plugin paths, secret env names, mounts, runtime stack mounts/overlays, workspaces, timeouts, max turns, sandbox session IDs, artifacts paths, and orchestrator/task context through the existing sandbox runner.
• Extends WordPress plugin smoke coverage for Homeboy-shaped input through the upstream runner contract.

Verification

• php -l packages/wordpress-plugin/src/class-wp-codebox-agent-sandbox-runner.php
• php -l packages/wordpress-plugin/src/class-wp-codebox-agent-task.php
• php -l tests/smoke-wordpress-plugin.php
• npm run build
• npm run wordpress-plugin-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Implemented the parent task runner adapter, CLI/schema updates, smoke coverage, and verification under Chris's requested issue scope.