Make CodingScaffold easier to install and run globally

[JRS1986]

Feedback

A tester noted that the current install path is friendly to Python developers, but painful for others:

1. Cloning the repo, creating a virtual environment, and installing from the checkout is too much ceremony for first-time users.
2. Installing inside the repo venv means coding-scaffold is only convenient after activating that venv, which is awkward when using it from another project folder.
3. A GUI could be useful later, but the CLI should first have a smoother install story.

Proposed scope

• Document an isolated global CLI install path, for example uv tool install or pipx install from the GitHub repo.
• Keep the clone + editable install path as contributor/development setup.
• Update generated onboarding text so scaffolded projects do not teach users to keep activating the source repo venv.
• Avoid adding runtime dependencies, network calls, or UI work.