Tighten OSS onboarding docs and remove the old git alias

[benvinegar]

Summary

• rewrite the README around install, quickstart, Git integration, and development
• add CONTRIBUTING.md and SECURITY.md for public-repo trust signals
• remove the never-shipped hunk git alias from the CLI, docs, and tests

Notes

• stacked on top of Prepare Hunk for npm packaging and install verification #8 so the docs can reference the new hunkdiff npm package name

Validation

• bun run typecheck
• bun test
• bun run test:tty-smoke