Summary

• Add explicit preview_only: true support to browser Playground sessions.
• Compile caller-declared runtime dependencies and prepared preview refs without resolving agent/provider inheritance or appending the PHP task runner.
• Keep browser task and materializer contracts agentic by default.

Contributes to #1743.

How to test

1. Run npm run test:php-browser-preview-only-session.
2. Confirm the preview-only blueprint includes its declared plugin, MU plugin, theme, bootstrap, and site artifact steps.
3. Confirm the preview-only blueprint contains no generated browser runner and its product DTO has a hydratable blueprint ref.
4. Confirm the default browser session still resolves provider inheritance and includes its runner.
5. Confirm create-browser-task-contract remains agentic even if a caller supplies preview_only.
6. Run npm run test:php-browser-contained-site-contract.
7. Run npm run test:browser-session-public-dto.
8. Run npm run test:browser-task-builder.
9. Run npm run build.

Backwards compatibility

This is an additive opt-in field. Existing browser sessions, task contracts, and materializer contracts preserve their agentic behavior.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode
• Model: openai/gpt-5.6-terra
• Used for: Implemented and verified an explicit preview-only browser session contract that omits agent/provider runtime setup; Chris reviewed and owns the change.