Split playground command handlers

[chubes4]

Summary

• Closes Split runtime-playground command handlers by responsibility #376.
• Split packages/runtime-playground/src/commands.ts into focused command-family modules for ability, bench, checks, arg parsing, PHPUnit, and WP-CLI helpers.
• Kept commands.ts as a compatibility re-export layer so existing command names, imports, inputs, outputs, artifacts, and error behavior stay unchanged.

Tests

• npm run build
• npm run command-registry-smoke
• npm run discovery-command-smoke
• npm run package-distribution-smoke
• npm run plugin-check-normalization-smoke (rerun serially after the first 120s tool timeout)
• npm run theme-check-normalization-smoke
• npm run core-phpunit-command-smoke
• npm run browser-probe-artifact-smoke
• npm run browser-actions-artifact-smoke
• npm run wp-codebox -- run --mount ./examples/simple-plugin:/wordpress/wp-content/plugins/simple-plugin --command wordpress.run-php --arg code-file=./examples/simple-plugin/probe.php --artifacts ./artifacts --json

Caveats

• This is intentionally a behavior-preserving extraction; the new modules keep the existing helper names exported through commands.ts for compatibility.
• npm install was required because the skipped-bootstrap worktree was missing dependencies. Incidental package-lock.json version churn from install was reverted and is not part of the diff.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Refactoring runtime-playground command helper modules, running verification, and drafting this PR body for Chris to review.