Extract runner workspace ability descriptors

[chubes4]

Summary

• Extract runner workspace ability registration descriptors into WP_Codebox_Runner_Workspace_Ability_Descriptors.
• Keep the ability hub responsible for registration order, aliases, permissions, and schema assembly.
• Update the adapter-boundary smoke assertion so it follows the descriptor extraction instead of requiring inline wp_register_ability() calls.

Tests

• php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php
• php -l packages/wordpress-plugin/src/class-wp-codebox-runner-workspace-ability-descriptors.php
• php -l scripts/php-runner-workspace-adapter-boundary-smoke.php
• npm run test:public-ability-aliases
• npm run test:php-runner-workspace-backend-contract
• php scripts/php-runner-workspace-adapter-boundary-smoke.php
• npm run test:php-public-api-facade
• git diff --check

Residual risks

• Runner workspace execution remains in WP_Codebox_Abilities_Runner_Publication; this PR intentionally stops at descriptor extraction to avoid broader behavior churn.
• npm run test:php-runner-workspace-adapter-boundary is not defined in package.json; the underlying PHP smoke was run directly.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Scoped descriptor/service extraction and verification