Separate browser task phase execution

[chubes4]

Summary

• Split browser task contract preparation from host-side phase execution so contract creation is side-effect-light by default.
• Added explicit execute_phases handling for fanout and host-delegation phase execution.
• Updated browser task schemas and smoke coverage for default preparation vs explicit phase execution.

Closes #781.

Testing

• php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-schemas.php && php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-execution.php && php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php && php -l tests/smoke-wordpress-plugin.php
• npm run wordpress-plugin-smoke
• npm run fanout-contract-smoke && npm run host-delegation-contract-smoke && npm run fanout-aggregation-contract-smoke && npm run agent-fanout-execution-smoke
• npm run build
• npm run check progressed through build, WordPress plugin smoke, and multiple check smoke scripts, then exceeded the 20-minute local tool timeout while running runtime-stack-mount-smoke.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the focused browser task boundary refactor and smoke test updates; Chris remains responsible for review and merge.