docs: add development setup instructions to README

[murrayju]

Summary

• Add a new Development section to README.md with setup instructions for contributors
• Include prerequisites (Bun, Node.js)
• Document setup steps and common development commands
• Cross-reference CLAUDE.md for detailed guidelines

Test plan

• Verified markdown formatting renders correctly
• All checks pass (typecheck, lint, test)
• README is accurate and helpful for new contributors

🤖 Generated with Claude Code

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[Copilot]

Pull request overview

Adds contributor-focused development setup documentation to the project README to make local setup and common workflows easier to discover.

Changes:

• Introduces a new Development section with prerequisites and setup steps
• Documents common commands for typechecking, linting/formatting, testing, building, and running from source
• Links to CLAUDE.md for more detailed contributor guidelines

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