Prepare isolated Guardian reviewer sessions (#40742)

## What changed

- Add a policy prompt and output contract for synchronous Guardian reviews.
- Build reviewer thread options with a preferred review model, low reasoning effort when supported, and a fallback to the parent model and effective effort.
- Isolate reviewer sessions from parent instructions, MCP servers, optional features, and write access while preserving read restrictions, resolved environments, managed network constraints, and rollout persistence.
- Mark prepared threads as internal Guardian review sessions for later host-managed startup.

## Testing

- Cover reviewer isolation, read-only environment inheritance, review-model overrides, and parent-model fallback behavior.

GitOrigin-RevId: 5133be99674b64e06e19e7f2dbc86439e1678790