feat: add readme-generator subagent

[ScientificAJ]

Summary

• add a new readme-generator subagent under 06-developer-experience
• update the main catalog and category README with the new agent entry and selection guidance
• bump the voltagent-dev-exp plugin version and sync the marketplace entry

Why

This adds a dedicated specialist for maintainer-ready README generation grounded directly in repository reality. It focuses on extracting exact setup commands, configuration, onboarding flows, and examples from the codebase with a zero-hallucination standard.

Overlap and distinction

This sits near documentation-engineer, but it is intentionally narrower and not redundant.

• documentation-engineer is broader documentation architecture and docs systems work
• readme-generator is README-first and repository-root-doc focused
• documentation-engineer is better for docs sites, multi-page docs, and larger documentation programs
• readme-generator exists to scan a repository deeply and produce accurate onboarding documentation from real source files, tests, scripts, and manifests