Refactor preview server helpers out of runtime index

[chubes4]

Summary

• Move preview proxy creation, header forwarding, preview-port availability checks, and local HTTP server helpers into preview-server.ts.
• Keep runtime boot and WP-CLI bridge orchestration in index.ts while delegating server/proxy plumbing.
• Preserve preview URL behavior, bridge JSON handling, and port-in-use error behavior.

Testing

• npm run build
• npm run command-registry-smoke
• npm run package-distribution-smoke
• npm run preview-port-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Identified and extracted the preview/local-server helper boundary, ran targeted verification, and drafted this PR summary. Chris remains responsible for review and merge.