Add browser task contract ability

[chubes4]

Summary

• Adds wp-codebox/create-browser-task-contract as a product-facing browser Playground contract that composes a primary browser session plus optional named phases.
• Supports generic materializer phases by reusing the existing browser materializer contract, sharing the parent session id by default, and preserving the disposable Playground/sandbox-bypass boundary.
• Extends plugin smoke coverage for registration, authorization, phase composition, session sharing, and unsupported phase rejection.

Refs #546.

Proposed API shape

{
  "goal": "Generate or transform a browser Playground artifact.",
  "authorization": {
    "schema": "wp-codebox/trusted-orchestrator-authorization/v1",
    "caller": "product-id",
    "scope": "browser-session:create"
  },
  "sandbox_session_id": "product-session-id",
  "runtime": {},
  "browser_runner": {},
  "phases": [
    {
      "name": "static-site-materializer",
      "kind": "materializer",
      "input": {
        "goal": "Materialize the generated artifact bundle.",
        "runtime": {},
        "browser_runner": {}
      }
    }
  ]
}

Returns wp-codebox/browser-task-contract/v1:

• primary: the normal wp-codebox/browser-playground-session/v1 response.
• phases[]: named phase envelopes; materializer phases contain the existing wp-codebox/browser-materializer-contract/v1 under contract.
• session: the primary session envelope for external orchestrator correlation.

Verification

• php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-execution.php
• php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-schemas.php
• php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php
• npm run build
• npm run wordpress-plugin-smoke (OK (369 assertions))

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the browser task contract ability, schema, smoke coverage, and verification steps for Chris to review.