Support bench bootstrap files

[chubes4]

Summary

• Add a bootstrap-files-json argument to wordpress.bench so callers can declare component-relative bootstrap fallback files.
• Load the first existing bootstrap file before synthetic plugin bootstrap hooks and before workloads execute.
• Add focused smoke coverage for the generated bench PHP contract.

Tests

• npm run build
• npm run bench-bootstrap-files-smoke
• npm run command-registry-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the runtime argument wiring, PHP bootstrap loading path, and focused smoke coverage; Chris remains responsible for review and merge.