Add Git repository import methods to the control plane

[norton120]

Why Do This?

Allow a project to be imported through either a one-time direct push or GitHub.

Definition of Done

• The control plane can generate a single-use authenticated remote URL that accepts exactly one git push to the target project's main branch.
• The URL becomes invalid after a successful push and cannot be used to push another branch or overwrite any branch other than main.
• A GitHub import path can pull a repository's main branch into the target project.
• Both methods report clear success and failure states, including authentication, missing repository, missing main, and rejected-push errors.
• Add automated coverage for both import methods and the single-use restriction.