docs: enhance README with badges, diagrams, query examples, and config reference

- Add shields.io badges (npm, license, downloads, build, node version)
- Add OpenCode link and compelling hero section
- Add mermaid diagram showing indexing/search flow
- Add 'When to Use What' decision table (grep vs semantic)
- Add good/bad query examples for better search results
- Add config options reference table with descriptions
- Add 'Tradeoffs' section with honest limitations
- Restore contributing guide and project structure
- Remove old test files from tracking (test-native.cjs/mjs)

Author: Helweg