Update CONTRIBUTING setup and test commands to match current README

[artfuldev]

CONTRIBUTING.md's setup and test/lint commands are inconsistent with what the rest of the repo (README, Makefile, AGENTS.md) actually documents and uses today.

Why did we do this?

Verified each command directly against this repo before changing anything:

• Step 3's rake db:create db:migrate db:seed and bare rails server still work once the native toolchain (mise, PostgreSQL, ImageMagick) is installed — but CONTRIBUTING.md never mentions installing that toolchain at all, unlike the README's own "Quick start", which walks through it. A contributor following only this file would hit pg gem compile errors with no explanation of why.
• Step 5's bare rake does still run the full RSpec suite (confirmed locally) — but serially, with no way to target files, and it's not what the project's own Makefile/README/AGENTS.md document or use (bundle exec rspec, bundle exec parallel_rspec).
• Step 8's bare rubocop happened to work in this session's environment because gems ended up resolvable outside Bundler — not something to rely on across different Ruby version managers. bundle exec rubocop is what the rest of the repo's docs consistently use.

None of these commands were actually broken; they were just out of step with the setup this repo has already standardized on elsewhere, which risks confusing a new contributor who reads this file expecting it to be the current source of truth.

Will this break anything?

No, this only touches documentation.

How was this tested?

Ran the corrected commands directly in this repo — real recorded terminal sessions (animated):

bundle exec rubocop:

bundle exec rspec (shown against one fast spec file for a readable capture; the full suite was also run and passes aside from pre-existing, unrelated local-environment gaps):

For setup, pointed at the README's own "Quick start" section instead of duplicating it here, so the two docs can't independently drift out of sync again.