Fix setup runtime resolution

[focusshifter]

Why

The development-environment task could fall back to macOS system Ruby because setup commands run in non-interactive login shells, while mise/asdf activation was written to the interactive shell RC file. This made bundle resolve to /usr/bin/bundle after the Factorial Ruby update.

The installer also duplicated the required Bundler version in Welcome, allowing it to drift from backend/Gemfile.lock.

What changed

• Execute Ruby and Node commands explicitly through the selected version manager.
• Read and install the exact BUNDLED WITH version from backend/Gemfile.lock.
• Avoid installing gems with the system Ruby or elevated privileges.
• Make dependency-install commands fail loudly.

Impact

Fresh setup and retry flows now use the repository-pinned runtimes even when the user's shell RC has not been loaded yet. Future Bundler upgrades require no matching constant update in Welcome.

The existing global @latest runtime installation remains unchanged so developers still receive current global defaults before the repository-specific versions are installed.

Validation

• npm run build
• npm run lint
• npm run format:check
• git diff --check
• Verified from a deliberately non-activated shell that unmanaged commands resolve to /usr/bin/ruby and /usr/bin/bundle, while mise exec resolves Ruby 3.4.8 and Bundler 2.5.11 from the Factorial repository.