Harden browser materialization runtime seams

[chubes4]

Summary

• Add a generic browser contained-site open-or-create ability for reusable/fallback Playground sessions.
• Enforce Playground command timeouts and add smoke coverage.
• Harden browser runtime agent bundle import, artifact-output guidance, and provider adapter credential/JSON handling for browser materialization flows.

Verification

• php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php && php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-execution.php && php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-browser-runner.php && php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-provider-adapter.php
• php tests/smoke-contained-site-ability.php
• npx tsx scripts/playground-command-timeout-smoke.ts
• npm run build

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Debugging browser materialization failures, drafting generic runtime/provider fixes, and adding focused smoke coverage.

[chubes4]

Superseded by the clean current-main replacement PR: #1260