Rewrite README for Pyrite — pyrite

The README.md still describes the upstream Zettelkasten project — credits "JoshChen", links to `github.com/joshylchen/zettelkasten`, and describes the Streamlit/CEQRC workflow. None of this reflects Pyrite.

Need a new README covering:

What Pyrite is (multi-KB research infrastructure)

Installation

Quick start (CLI, MCP, API)

Plugin/extension system

Architecture overview (link to kb/designs/)

Contributing

Rewrite README for Pyritebacklog_item

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