Add prd-maker to Developer Productivity

[jcmaker]

Adds prd-maker under Developer Productivity.

What it is: an interview-driven PRD generator. It asks about your product idea (adapting the questions to your technical level) and produces a PRD.md that an AI coding agent can execute directly.

Why it fits this list:

• It's a Claude Code plugin (.claude-plugin/), and also ships a Codex plugin manifest — cross-agent.
• Genuine use case: turning a vague idea into an agent-executable spec, with explicit non-goals, machine-verifiable acceptance criteria, and phased scope.
• Not a duplicate — I searched the README and there's no existing PRD / spec-generation plugin.
• Tested: CI runs a linter test suite + structure/manifest checks on every PR (badge in the repo).

Entry:
- [prd-maker](https://github.com/jcmaker/prd-maker) - Turns a product idea into an agent-executable PRD.md through an adaptive interview. Works for non-developers, outputs pure markdown, and supports Claude Code and Codex.

Thanks for maintaining the list!