docs: add CONTRIBUTING.md with development and PR workflow guide

[okwn]

Summary

Adds a CONTRIBUTING.md guide covering:

• Development setup — prerequisites (GHCup, Docker, Node.js 22, LLVM, libpq), initial clone and git hook setup
• Running locally — , optional profiles (dev, test), environment variables
• Building from source — , live-reload with ghcid --command 'cabal repl monoscope --ghc-options="-Wno-error=unused-imports -Wno-error=unused-top-binds" --with-compiler=ghc-9.12.2' --test ':run Start.startApp' --warnings
• Running tests — unit tests (), doctests (), integration tests (), Makefile aliases
• Code style — Fourmolu auto-commits on push (no manual formatting needed), HLint linting and auto-fix, Prettier for frontend
• PR workflow — fork & branch conventions, making changes, auto-formatting behavior on push, opening a PR
• Project structure overview

Files Changed

• (new) — 253 lines

[tonyalaribe]

Thanks for the PR @okwn