Refactor runtime WP-CLI bridge helpers

[chubes4]

Summary

• Extract runtime WP-CLI bridge request handling and command splitting into runtime-wp-cli-bridge.ts.
• Keep PlaygroundRuntime responsible for the existing VFS-backed WP-CLI command runner while delegating bridge transport details.

Testing

• npm install (fresh worktree dependency install; prepare build passed)
• npm run build
• npm run command-registry-smoke
• npm run package-distribution-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 focused extraction and ran verification; Chris remains responsible for review and merge.