Harden browser runtime materialization seams

[chubes4]

Summary

• Enforce timeoutMs for Playground runtime commands.
• Preserve browser task agent_bundles when running compact browser-session recipes.
• Improve generic browser runtime agent-bundle importer loading/diagnostics.
• Add generic artifact-contract output instructions to browser runtime agent messages.
• Normalize JSON bodies/content type in the generic browser provider bridge.

Verification

• npm run build
• npx tsx scripts/playground-command-timeout-smoke.ts
• npm run smoke -- --group=runtime
• php scripts/php-browser-contained-site-contract-smoke.php
• php -l packages/wordpress-plugin/src/class-wp-codebox-agent-runtime-invoker.php
• php -l packages/wordpress-plugin/src/class-wp-codebox-browser-provider-bridge.php

Supersedes conflicted PR: #1259

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Porting generic browser runtime hardening to current main, resolving conflicts from refactored runtime code, and running focused verification.