Add browser materialization capture contract

[chubes4]

Summary

• Adds browser_runner.capture_paths to the browser Playground session contract so callers can ask Codebox to capture sandbox-local output files or reports after invocation.
• Normalizes generated browser runner results into wp-codebox/browser-materialization/v1 with invocation metadata, captured file records, and structured errors.
• Documents the generic capture contract and extends the WordPress plugin smoke coverage.

Closes #438.

Testing

• php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php
• git diff --check
• npm run wordpress-plugin-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the generic browser materialization invocation/capture contract, documentation, and smoke coverage under Chris's requested issue scope.