Add in-process REST request runtime primitive

[chubes4]

Summary

• Add wordpress.rest-request, a generic Playground runtime command that executes WP_REST_Request through rest_do_request() inside the WordPress process.
• Add a rest_request runtime action adapter mapping for episode traces and observations.
• Add smoke coverage for direct command execution and runtime action usage against /wp/v2/types.

Fixes #440.

Verification

• npm run build
• npm run rest-request-runtime-smoke
• npm run command-registry-smoke
• npm run runtime-action-adapter-smoke
• npm run runtime-episode-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the implementation, smoke coverage, and verification commands for Chris to review.